Unhairing process by oxidation with hair recovery

Introduction Given the amount of legislation on the environment, tanners are required to reduce all kinds of pollution (solid wastes, dissolved or suspended substances in effluents etc). It is well known that beamhouse operations cause the highest pollution during the leather manufacturing process. Data provided by the Centre Technique du Cuir¹ show that, in the beamhouse, the conventional unhairing process with sodium sulfide and lime is responsible for 84% of the BOD5, 75% of the COD and 92% of the suspended solids. Consequently, the replacement of the conventional unhairing process with sodium sulfide and lime by a less polluting procedure is of paramount importance to the tanner. Several alternatives to the conventional process have been studied. These include the unhairing of hides by oxidation. This has been looked at by several authors^2-5. Recently a number of studies have been carried out to show the possibilities offered by the use of hydrogen peroxide in a highly alkaline medium^6-11. The results obtained by this unhairing method are satisfactory; it causes hair destruction which resulted in highly contaminated wastewater. The application of an oxidising unhairing process with hair recovery would allow the replacement of sulfide and the reduction of the pollution load of the resulting wastewater. However, it is necessary to immunise the hair against the subsequent action of H₂O₂ in an alkaline medium. In previous work¹², the immunisation capacity of Ca(OH)₂ alone or in combination with Na(OH) - on mechanically separated hair from goatskins - was studied against the oxidising attack of H₂O₂ in an alkaline medium. Unhairing with hydrogen peroxide in alkaline (without hair recovery) Studies have revealed that a pH of 12.5 and an H₂O₂ offer of 8% (on fleshed weight) were necessary for the hides to be unhaired satisfactorily. Under these conditions, the hair is destroyed. An additional operation of the wastewater consisting of protein precipitation by acidification with H₂SO₄ down to pH3.5 and subsequent protein separation by filtration allows the filtered water to be recycled, leading to a reduction in water consumption. It was also shown that hides unhaired with hydrogen peroxide slipped more in the fleshing and splitting machines than those unhaired with sodium sulfide and lime. The joint effect of amines and hydrogen peroxide in an alkaline medium on the unhairing of hides gave rise to a better mechanical action when fleshed and split than hides unhaired with hydrogen peroxide alone. Likewise, the addition of the amine product (0.8% on fleshed weight) allowed a reduction in the hydrogen peroxide offer (6% on fleshed weight) when compared with hides unhaired with hydrogen peroxide alone (8%). This study also revealed that leathers unhaired by the oxidisation process with H₂O₂ and amines showed lower physical property values (tensile strength and tearing load) than leathers treated with the conventional unhairing process. The reducing character of the conventional unhairing is believed to have caused less chromium in the tanning operation. Another study contradictorily revealed that the mechanical resistance diminished almost linearly as the leather's Cr₂O₃ content increased regardless of the unhairing method applied. Leathers with a similar Cr₂O₃ content, ie equal mechanical resistances, were produced by reducing the offer of chromium tanning salt for pelts unhaired with the oxidising process. This lower offer allows a saving, albeit a small one, in chromium salt and a potential reduction in the chromium content of the wastewater. Objectives The results of previous research, as summarised, constitute the starting point of the present study. However, the best process conditions determined are still too aggressive as far as the sodium hydroxide and hydrogen peroxide offers are concerned since they cause hair destruction. The aims of the current work were to determine the best process conditions, which allowed hair recovery. In this case, it was necessary to protect the hair against the subsequent action of H2O2 in an alkaline medium. However, it is not convenient to achieve a total immunisation of the hair since it would then be impossible to remove. The influence of a calcium hydroxide offer on the immunisation process was looked at. Likewise, the effect of other variables such as sodium hydroxide and hydrogen peroxide, the process time and the addition of auxiliaries on the oxidising unhairing process were considered. The feasibility of the unhairing process as a function of the above-mentioned variables, the quality of the recovered hair and the level of the pollution load of the resulting wastewater are the major parameters for evaluation. Experimental Raw materials and chemicals Salted hides were received from the tannery after soaking and fleshing. They were cut in 20cm x 20cm samples which were subjected to the unhairing process under a number of varying conditions. The following chemicals (industrial grade) were employed in the unhairing processes: * Hydrogen peroxide solution (35%) * Commercial chemical based on secondary amines (Ribersal PLE, ex-Cromogenia Units) * Commercial bacterial protease (Riberzym MPX, ex-Cromogenia Units) The chemicals employed in subsequent operations were those typically used in the leather industry. Starting formulation Table 1 shows the starting formulation. The variables considered in this formulation were sodium hydroxide and hydrogen peroxide, a calcium hydroxide in the immunisation process, the process time and an enzyme addition. Unhairing process evaluation The unhairing processes were carried out under a number of variables and were evaluated by considering the way in which the hair was eliminated. In those cases where the hair could be recovered, its characteristics were evaluated by considering its appearance as well as the cystine content in accordance with the method outlined in the box: Influence of sodium hydroxide The formulation shown in Table 1 was applied with the following parameters: x = calcium hydroxide offer in the immunisation process (1.5%) z = hydrogen peroxide offer in the unhairing operation (3.0%) t = unhairing process time (20 hours) The sodium hydroxide offer in the unhairing phase was studied at four levels: 0, 2, 4 and 6% on fleshed weight. Table 2 shows the results obtained. As observed in Table 2, a minimum offer of sodium hydroxide is required for a complete hair elimination. For a hydrogen peroxide offer of 3%, this minimum offer of NaOH is 4%, although in this case - and also for the highest NaOH offer (6%) - the hair is destroyed. In those cases where the hair elimination is not complete and hair can be partially recovered (treatments 1 and 2), the highest loss of cystine corresponded to the sodium hydroxide offer (treatment 2). Influence of hydrogen peroxide The influence of the hydrogen peroxide was carried out for two sodium hydroxide offers: 2% and 4%. With respect to the formulation shown in Table 1, the following parameters were kept constant: x = calcium hydroxide offer in the immunisation process (1.5%) t = unhairing process time (20 hrs) z = sodium hydroxide offer (2 or 4%) The effect of the H₂O₂ offer was considered at five levels: 0, 1, 2, 3 and 4%. Table 3 shows the results obtained for a NaOH offer of 2%. As can be observed, hair elimination is not complete for any H₂O₂ offer. Probably, with this level of NaOH, the minimum pH value required for complete unhairing has not been reached. In all the treatments, hair can be partially recovered. The hair is more degraded with increasing H₂O₂ amounts. Results for a NaOH offer of 4% can be observed in Table 4. For this amount of sodium hydroxide, a complete hair elimination took place whatever the hydrogen peroxide offer was. Hair could be recovered for a hydrogen peroxide dosage up to 2%. Higher amounts of H₂O₂ resulted in hair destruction. Increases in the swelling effect were observed in pelts unhaired with the lowest H₂O₂ offer (1%) which corresponded to the highest pH. This increased swelling gave rise to grain damage. Influence of process time The starting formulation shown in Table 1 was applied. The following parameters were kept constant: x = calcium hydroxide offer in the immunisation process (1.5%) y = sodium hydroxide offer (3%) The hydrogen peroxide offer was studied at two levels: 0 and 2%, and the unhairing process took place over 10 or 20 hours. The results obtained are shown in Table 5. As shown, the process time of 10 hours was not sufficient for a complete hair elimination. For 20 hours, the treatment with 3% NaOH (treatment 16) showed some hair loosening which could be mechanically removed afterwards. However, some hair roots remained. In process 18 (3% NaOH and 2% H₂O₂), hair was eliminated during the process so that unhairing was almost complete, however, some hair roots remained on the surface, although less than those seen in treatment 16. The influence of the unhairing process time should be investigated further to be optimised. Influence of immunisation In all the treatments carried out up to now, the immunisation process was performed with 1.5% of calcium hydroxide. In the next phase of the study, the results of varying the Ca(OH)2 amount were investigated. The Ca(OH)2 offer was added at four levels: 0, 0.5, 1.0 and 1.5%. The initial formulation was applied, keeping the following parameters constant: y = sodium hydroxide offer (4.0%) z = hydrogen peroxiode offer in the unhairing operation (2.0%) t = unhairing process time (20 hours) Table 6 shows the results obtained. Hair elimination was complete in the four treatments used. However, hair could not be recovered (it was destroyed) in treatment 19 where immunisation was not carried out. So, immunisation with calcium hydroxide allows the hair to be recovered, as stated in previous studies¹³. Nevertheless, the calcium hydroxide offer is of paramount importance. In treatment 20, the low calcium hydroxide offer results in a lower amount of hair recovered, since it is partially destroyed, whereas in treatment 22, with highest level of calcium hydroxide in the immunisation process, hair removal is slower since the hair is strongly immunised. In this case, hair is removed during the acidification and any washings carried out afterwards. As far as the cystine content is concerned, the highest calcium hydroxide offer in the immunisation process gave the highest protection against the subsequent attack of hydrogen peroxide in an alkaline medium. Influence of enzymes The influence of a bacterial protease (0.15% on fleshed weight) added after the unhairing process was studied. The following parameters were kept constant in the starting formulation: x = calcium hydroxide offer in the immunisation process (1.0%) y = sodium hydroxide offer (4.0%) z = hydrogen peroxide offer in the unhairing operation (2.0%) t = unhairing process time (20 hours) The addition of the enzyme resulted in the production of cleaner pelts and a reduction in time required for unhairing. However, a slight loosening of the grain with wrinkling was observed. The influence of enzymes on the oxidising unhairing process with hair recovery still requires further investigation. Evaluation of the pollution load From the results obtained so far, it can be concluded that the best conditions for the unhairing with hydrogen peroxide in an alkaline medium with hair recovery, on a pilot plant scale (applying in formulation shown in Table 1) are the following: x = calcium hydroxide offer in the immunsation process (1.0%) y = sodium hydroxide offer (4.0%) z = hydrogen peroxide offer in the unhairing operation (2.0%) t = unhairing process time (20 hours) The COD of wastewater following the unhairing process, applying these conditions, was determined as a measure of the pollution level produced. A value of 9,200ppm O₂ was obtained. The evaluation of this figure should be related to the water consumption of the unhairing process (including the 'amine pretreatment') which was 2.5l/kg of soaked and fleshed hide. Conclusions From the results of this work carried out on a pilot plant scale, the following conclusions can be drawn: The possible application of an oxidising unhairing process with hair recovery as an alternative to the conventional process was confirmed. The immunisation with calcium hydroxide prior to the subsequent attack with sodium hydroxide and hydrogen peroxide allows the hair to be recovered. A calcium hydroxide offer of 1% (on fleshed weight) results in a sufficient immunisation of the hair. A certain level of sodium hydroxide, ie a minimum pH value, is necessary to achieve a complete hair elimination. This minimum NaOH dosage corresponds to 4% on fleshed weight. For this NaOH amount, an hydrogen peroxide offer of 2% is sufficient for unhairing and allows the hair to be recovered. Increasing H₂O₂ offers result in hair destruction. The process conditions obtained in this work must be obtimised when applied on an industrial scale. Acknowledgements The authors are indebted to the Comision Interministerial de Ciencia y Tecnologia (CICYT) for the financial support given through the project AMB98-0868. A version of this paper was originally presented at the XXVI IULTCS Congress held in Cape Town, South Africa, March 7-10, 2001.