A novel approach14 June 2019
Jin Li-qiang and associates from Biosk Chemicals discuss the results of a study into recycling technology for unhairing-liming and tanning waste water.
Recycling technology is one of the most important ways to realise clean production in the leather industry. In this study, a novel closed recycling technology of unhairing-liming and chrome tanning waste water has been applied in mass production scale in a well-known local Chinese tannery. In the recycling process, the content of various ions and total organic carbon (TOC) of the waste water were measured. The collagen fibre dispersion and the distribution of ions in pelts were characterised, and the shrinkage temperature and mechanical properties of leather obtained by the recycling technology were also tested.
All the trials showed that the novel closed recycling technology worked well without impairing the quality of the resultant leather, and that zero emissions of unhairing-liming and chrome tanning waste water were realised. In the unhairing-liming step of the novel technology, the dosage of water, sulphide and lime was reduced by 80.8%, 33.3% and 23.4%, respectively. The collagen fibre bundles of limed pelts were split well. The distribution of calcium (Ca) ion and sulfion in limed pelts was more uniform.
As much as 70% of salt and 20% of chrome tanning agents could be saved by recycling chrome tanning waste water. The shrinkage temperature of wet-blue from different recycling times (from the first to the 30th) was all above 100°C. With increased recycling times, the grain surface colour of the wet-blue tended to be similar. Furthermore, the mechanical properties and yield of the leather in the recycling technology were improved.
Closed cycle technology
Unhairing-liming and chrome tanning are two of the most important steps in the production process of leather making. The quality of the unhairing-liming and tanning process will affect the following process as well as the properties of the finished leather. The traditional unhairing-liming waste water contains large amounts of sulphide, lime, protein and its degradation product. The tanning waste water contains chrome complex and neutral salt, and therefore the unhairing-liming and chrome tanning waste water is the main source of pollution in leather manufacturing. Furthermore, the terminal control method is usually used in the liming and chrome tanning waste water treatment, but the treatment fee is high and the treatment system is not very effective.
The recovery and reuse of waste water is one of the effective ways to realise the clean production of liming and chrome tanning, which has been paid more and more attention by researchers. In recent years, several patents have been granted to chemicals company Biosk related to closed cycle technology of liming and chrome tanning effluent.
According to the principle of this technology from Biosk, the closed cycle technology of unhairing-liming and chrome tanning waste water in the production of cattle shoe leather was investigated.
The aim of this research is to explore the technical points of the recycling technology of liming and tanning waste water, and evaluate the effect of the process and the quality of the obtained products.
Chemicals and processes
The chemicals used in the experiment were as follows: Na2S, NaHS, lime and chrome powder were industrial grade. Degrease agent AN-C was supplied by Lanxess. The liming auxiliary DOpro and ELIPO, fungicide DK and basifying agent BE were supplied by Biosk, and lime dispersing auxiliary CR-2 was purchased from American BK Company. Also, the dosage of chemicals was calculated according to the weight of salting hide.
The waste water of the experimental process was collected and then reused in the next production batch. Because of the restriction of the experimental conditions, only 80% of the liming waste water was collected and a certain amount of sodium sulphide as well as the lime was supplemented in each recycling.
The chrome contained in waste water from the experimental process was collected and then reused in the next production batch for pickling and tanning. Due to the limitations of the experimental conditions, only 90% of chrome tanning effluent was collected.
The sample of liming waste water was first precipitated. After natural sedimentation, 500ml of the waste water of different recycling technology was filtered twice by double layer gauze. The filter liquor was then taken for analysis, and the content of sulphide in the waste water was determined by filtration and iodimetry.
The leather sample was then ashed at 550°C and the 1:1 hydrochloric acid was used to resolve the ash, and then the sample was diluted. The titration process was similar to the determination of Ca in the waste water.
Total organic carbon levels
The TOC was detected by a TOC analyser and the liming liquid waste was diluted 500 times and filtered through filter membrane (0.45μm) before analysis. The buffer capacity of the liming liquid waste was detected by an automatic potentiometric titrator and the TOC of chrome tanning waste water was detected as mentioned. The Ts of wet-blue and the content of Cr2O3 in waste water were also determined in accordance with Chinese standards.
Approximate volume in square feet of wet-blue under Leather Working Group certification.
Leather Working Group
The limed pelt was dehydrated four times overnight by 100ml ethanol and drying for 24 hours at 40°C. The grain and vertical section of the dried samples were analysed by super depth of a field microscope and by environment scanning electron microscope. The tensile strength, bursting strength, tear strength, contents of Cr2O3 and other related properties of the finished leather were also tested according to further Chinese industrial standards.
Pelt and waste water characteristics
In this recycling experiment, the weight of leather was about 10t every time. The dosage of water, sulphide and lime was reduced by 80.8%, 33.3% and 23.4%, respectively, and in the whole process, the waste water discharge was zero. Compared with the normal craft, the pelt obtained by this recycling technology was evenly swelled. The pelts also showed a clean surface and an obvious weight gain.
The sulphide content of liming waste water in normal process was higher than 5g/L. The Na2S content of waste water at a different time in the recycling process was in the range of 4–6g/L. According to the results, the Ca content of waste water in the normal process was the highest, about 2.25g/L. In the following recycling technology, the Ca content in wastewater was in the range of 1.87– 2.03g/L. The rangeability is small. The variation of Ca content in the different recycling process indicated that the recycling technology not only decreased the dosage of lime, but also decreased the residue content in the waste water. Compared with the conventional liming process, the Ca content of pelts in different recycling times decreased a little.
In order to analyse the content of organic material in the waste water, the TOC was determined in order to characterise the pollution of organic material in the waste water. The TOC of waste water from the conventional method was 14,750mg/L. This was due to the dissolution of the interfibrillar substance in the liming process. In the liming process, the interfibrillar substance as well as part of the collagen fibre would dissolve in the liming waste water. Therefore, the content of organic material in waste water would increase. At the same time, the collagen fibre was dispersed sufficiently.
The amount it is possible to decrease COD and BOD5 by when applying methods such as hair-save liming.
With the increase in recycling time, the TOC of waste water increased due to the dissolution of organic material. After recycling four times, the TOC of waste water tended to be constant; furthermore, the viscosity of liming waste water did not tend to thicken. These might be attributed to the hairpersevering technology. In the hair filter process, part of the organic material would be absorbed into the shedding hair and then filtered out by the recycling system – so the recycling process could be carried out.
The estimated amount of all leathers in the world that are chromium III tanned.
Calcium content and distribution
The solubility of lime is limited, so the pH of saturated lime solution is in the range of 12–13. The mild liming condition makes the alkaline swelling slow and uniform. The operable and safety of lime swelling are attributed to the chemical property of lime.
According to results, the titration curves of Ca(OH)2 solution had an obvious pH abrupt point. On the contrary, no obvious abrupt point appeared in the titration curves of liming waste water. So the liming waste water displayed the outstanding buffering ability. This might be attributed to the existence of degradation products of the interfibrillar substance in waste water. Polysaccharose substance and protein showed a good buffering property, so the pH of the liming waste water changed slowly when the acid was added. In the liming recycling system, the buffering properties of the liming waste water would slow down the swelling rate of the rawhide. Furthermore, the occurrence of liming wrinkle in the liming pelt decreased. Then the obtained pelt became smoother and the yield of crust increased.
The morphology of grain of limed pelt was investigated in super depth by the field microscope. The results indicated that the hair of two pelts was removed thoroughly and the hair pores were both clearly visible. This meant the hair root was also removed thoroughly. So the recycling technology of liming wastewater was safe and reliable.
Indications show that the Ca content of the grain layer and flesh side was higher than that of the middle layer. After being recycled four times, the distribution of Ca was uniform in the whole vertical section of limed pelt. This might be attributed to the existence of a large amount of organic material in the recycling liming waste water. The swelling process was assuasive but the liming process was more uniform. As a result, the penetration of Ca was better. The ‘S’ distribution in the samples from the conventional technology and the fourth recycling process were similar. This might be attributed to the solubility of sulphide since the penetration property of sulphide was higher than lime.
The Ca distribution in the pelt of the grain layer, middle layer and flesh side was 37.29%, 6.14% and 27.97%, respectively. Compared with the normal craft, the Ca distribution of pelt obtained by recycling craft in the grain layer, middle layer and flesh side was 29.33%, 12.95% and 23.91%, respectively.
Novel recycling technology evaluated
The Ts of wet-blue obtained by different recycling times and in the range of 101–107°C was determined and didn’t change so much with the increase of recycling time. The colour of the grain surface of wet-blue was consistent with the increase of recycling time. The content of TOC and Cr2O3 in the recycling waste water grew with the increase in the recycling time until the 10th time it was recycled. Then, the total organic compounds and remaining Cr component in the tanning waste water reached equilibrium.
In order to assess the novel recycling technology, the physical mechanical properties of the finished leather samples obtained by conventional process and the recycling technology were compared. The results indicated that the physical mechanical properties of finished leather were not influenced by the recycling technology. On the contrary, the tensile strength and tear strength of leather samples from the recycling technology were better than that of the sample obtained by the conventional technology. Furthermore, the yield of leather in the recycling technology was 95%, while the conventional technology is about 93%. This may be attributed to the uniform swelling process of the collagen fibre in the recycling of liming waste water, and the loss of the collagen content was less.
Performance and results
Waste water recycling technology was applied and the zero discharge of liming and chrome tanning waste water was realised. Compared with the normal process, the novel recycling technology could save more than 80.8% water, 33.3% of sulphide and 23.4% lime in the liming process. In the chrome tanning waste water recycling craft, 20% of the chrome tanning agent as well as 70% of salt were saved.
In the liming waste water recycling process, the reduction of the dosage of lime and sulphide didn’t affect the removing efficiency of hair. The distribution of lime in the limed pelt was more uniform. In the recycling process, the special auxiliary can prevent the deterioration of the chrome tanning waste water.
The physical and mechanical properties of the leather sample obtained by recycling craft were similar to the sample obtained by the normal craft. In addition, the yield and the chrome content in the leather sample obtained by recycling craft are higher than the leather sample obtained by the conventional process.