RECYCLING OF TANNERY LIQUORS*

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Preston Derek Gibbs
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1 + Johrnal of the Society of Leather Technologists and Chetnists, Vol. 61, p. 63 RECYCLNG OF TANNERY LQUORS* F. van TORNOUT Centre Technique du Cuir, France t is a well-known fact that the leather industry, especially in the developed countries, has been put under stress by authorities who cover the ecological problems. t is not my aim to review the tremendous amount of work that has been done giving details of how much the tannery may be a polluting business as seen from the local environment. The first aim of a tannery is not to produce drinkable water but a piece of high-quality leather which takes part on a national level as a positive socio-economical activity. The question is this: how we get our quality leather and at what price? Roughly a price can be split up into: hides or skins, labour, equipment, energy, chemicals, pollution rates, benefit or loss. can see at least two items in this series where the tanner can tighten the screw: chemicals and pollution, and both are linked to each other. t is relevant that waste chemicals will be found in the pollution if no preventive action is undertaken. The bill for pollution can go from rather important to suicide. Hence, we may try to look at how far the anti-economy produced by wasted chemicals and pollution of a factory can be reconverted into a less negative or even a slight positive factor in your price structure. 1. The First point is to Save Chemicals by Avoiding Using an Excess at the Start 1.1. The first chemical in a tannery is wwtrr and normally we pay for the volume we put into our process. We have an interest in working with reduced quantities of water wherever it is possible. Reduction of bath lengths and, especially, replacing rinsing operations by discontinuous washings are the most effective means to save water. The rinsing or washing operation is called efficient when those products are eliminated from the hide that we do not want any more. We know that the penetration speed of products into the skin is rather slow and drumming is a common means to speed up the absorption of chemicals. We should not forget that practically the same phenomenon takes place at the rinsing or washing stage but in the opposite way. Undesired products leave the pelt slowly and any quantity of water is well used if it has the opportunity to take up the products we want to extract from the skin up to aconcentrationwhichisroughly in equilibrium with the one we have inside the skin. * Presented to the South African Convention Rinsing over a length of time and with very important quantities of water can achieve the elimination of the undesired products but the water is spoilt since at any moment the concentration of extracted products in the water phase is far below equilibrium. Washing is able to cut down considerably the time and water consumption during the operation as the concentration of extracted products in the washing water reaches a high level. n fact, we should remember how we have washed precipitates while running some inorganic analysis at school learning chemistry that should be applied to tannery. t has been proved that by applying batch washing in place of rinsing, we can save 50 '%, of the water consumption> used in a classical operation. Water saving is an important job, to be done in every tannery, and by the way, we should not forget that some of the water we use must be heated, hence calling for 1 kcal each time we will raise the temperature by "c/litre of water t is often stated that too much lime, sodium sulphide, chrome salts, dyestuffs, etc., are used for the sake of mystery but not corresponding to a technical need. t should be a permanent charge for technical staff in the tannery to alter traditions by looking at rather the technical background of the processes. Analytical control of leathers and exhaustion rates of spent baths are good tools to serve this purpose. 2. Having Reduced the nput of Chemicals to a Strict Minimum We can Discuss the Preventive Treatments for Waste Liquors f the pollution disposed of by the tannery forms a coherent and relatively well-defined whole, the contribution of each of the liquors is extremely diverse and depends on the operation under consideration. Table Soaking Liming Deliming and Tannage % %, Hating % ROD COD Salt 60 x Suspended solids 5 55 Toxicity 80 5 n a schematic way, Table 1 reviews the distribution of pollution parameters according to the different phases of leather manufacture. From this table, we learn that liming is the operation which produces as much as half the total pollution and that soaking gives rise to 60% of the salt pollution. What has not been represented in this table is the toxicity due to waste solvents from degreasing operations which have a high toxicity. Hence, small quantities of paraffinic solvents present in an effluent can.

2 disturb completely the aerobic digestion in a treatment plant. n order to prevent an important amount of this pollution, two types of treatments can be considered: first: the recovery of products by separating techniques; second: the direct recycling of waste liquors Separating Techniques Applied to Waste Lime Liquors Separation cd sodiutn.supiiclc from hpent limc liquors is not a new technique, the principle of which is to strip hydrogen sulphide from the spent liquor by acidification of the solution. The reaction is carried out in a gas-tight room. The clarified solution is stirred while acidified and the produced ga5 flows in countercurrent to a caustic soda solution in order to react and produce a solution of Na sulphide. Acidification brings about a precipitation of some proteins present in the liquor. After settling this sludge can join the sludgetreatment circuit of the factory. Table 1 Na&+ HnS04 -+ Naas04+H~S HzS + 2Na(OH) --t NaZS + 2H20 HzS+NaOH --f NaHS+HzO HzS+NanS + 2NaHS This method allows 90% of the NaeS present in the spent liquor to be recovered which represents 40-50% of the initial input. some Fench factories are still running this technique Separation of Na2S and recovery of proteins from spent lime liquor. This technique has been described by Blazej. The principle again is acidification of the spent liquor, capture of the stipped H2S in caustic soda, and treatment of the precipitated proteins in order to obtain cattle-food. n fact, the major quantity of proteins (30-40 kg) of dry matter per ton of salted hides are present in spent lime liquors. The proteins are essentially keratinic, containing at least 10% of sulphured amino acids. The recovery of these proteins has interest since they possess nutritive value and good digestibility. From a technico-economical point of view, this process is characterised by: a 70% recovery of the volume of the liquor; a 50% economy in sodium sulphide; absolutely no effluent from spent lime liquors Separation of Na2S and proteins by ultrafiltration. n the previous case, the isolation of the proteins was only possible after modifying their appearance : the molecules in solution were transformed into aggregates of sufficient size and mass to be treated with a classical method of separation. Among these, filtration is perhaps the oldest. To satisfy the demand to isolate finer and finer particles, more specific filtering screens with more separating efficiency were sought. Twentieth-century chemistry has allowed the synthesis and development of polymers which, suitably adapted, constitute permeable films of a certain selectivity with regard to the constituents of the solution in which they are placed. They are called artificial membranes. 64 Membranes proposed for ultrafiltration are characterised in this way that they are pcrmcurhlc to solvent A (for instance water), dissolved matter with small molecular size (for instance salts) and impermeable to dissolved S, the molecular weight of which is high. By exerting an appropriate pressure P, a binary permeated liquid A+s is eliminated and the concentration of S in the retained liquid is increased. This technique has already numerous applications: pharmocnrtiral inclristr)~ (isolation, concentration and purification of proteins, preparation of vaccines), food industry (concentration of milk). This technique is now under study at CTC. We are trying to apply ultrafiltration to spent lime liquors with the aim of recovering Na& dissolved lime, proteins, and finally water. Through the actions of the specific membranes, resistant to the highly alkaline ph of the lime liquor, the spent liquor being screened before it is pumped on the filter cell, is divided in two parts: the ultrafiltrate contains water, Na& dissolved salts (chloride) and the degraded protein molecules small enough to pass through the membrane, a concentrate composed of detained protein sludge, undissolved lime, the whole impregnated with salt solution. The ultraliltrate is stored in a tank where it will be reinforced with water lime and sulphide before its recycling in the leather process. The concentrate is reinjected into tank so as to enrich its content in organic matter; when its concentration is sufficient, the protein sludge is precipitated, desalted and dried. We have been running batches of 20 m3 spent liquor and the yield of such an operation is as follows: Recovery of 40 o/, of sodium sulphide 20% of lime of liquid volume. These figures mean a reduction of four-fifths of the toxicity of the effluent and a potential commercialisation of kg keratinic protein/ton of salted hides. These proteins have a digestibility of 75 x. As we will see later this new technique cannot challenge the direct recycling of spent lime liquor as the investments are important unless new fields of applications for the keratinic proteins can be found. We are looking in this direction but up to now no relevant results can be reported. Recycling of Spent Lime Liquor The principle of a recycling process consists of recovering the total spent bath and to reuse it without any treatment, except the reinforcing to its original concentration in active chemicals. Nevertheless, it must be indicated that in this case a spent lime liquor contains solid waste, for instance pieces of hide, and non-destructed hairs so that screening is necessary. We have been working on a vibrating screen with a mesh of 1 mm. Liming was done each time on a 50 kg batch (two light cow hides cut in sides). After soaking for 24 h the hides have been fleshed, giving a weight upon which the chemicals to be used are calculated. The procedure was as follows: (lime painting using 30% of the lime liquor volume): of technical; 3 NarS; 2';G lime.

3 ' 1 After hair loosening, the hides are allowed to swell adding 150% of the lime bath in three partial additions. The total time was 20 h. The reinforcings along the whole series of recyclings were calculated to maintain those proportions. Layout of the Problem During liming, a certain amount of sulphide and lime are consumed, while a fraction of the liquor with the dissolved products is detained in the hides as a result of the alkaline swelling. So, some of the original constituents disappear from the bath. n parallel, other phenomena occur: degraded hairs are found or are solubilised or in suspension in the spent liquor; starting with salt cured hides some of the residual salt content migrates into the lime liquor; and finally, some residual fats even after fleshing pass into the lime bath. All these facts make it evident that the composition of a spent lime liquor is rather a complex mixture and analysis results in the following figures calculated on the fleshed weight. Table Composition nitial Residual NazS 3% 1.5% Ca(CH)z 2% 1.2% Nitrogen % NaCl % Grease % Solids Water 5% 180% 7% 140% PH From this table, we state a loss of of the bath (absorption in the hides, loss on screening and pumping); 500/,of thenazsand60%of the 1ime.Takingintoaccount the reduction of volume, the partial consumption of chemicals, an increase to 140% of dry residue gives evidence that an important load of specially organic matter (up to 3.2% on fleshed weight) is present in the liquor. The reuse of the bath should lead to a steady increase in all sorts of residues preventing, in the long run, the action of NazS and lime on the hair. Only a fair experimental approach can bring the answer. We have recycled a liquor 18 times over a period of two months. The evolution of different essential factors or the system have been followed for three reasons : 1. The unhairing efficiency as well the liming effect must be guaranteed on each batch of hides. 2. n a pilot experiment, a high amount of analysis can be taken into account but this will be impossible in an industrial application. Only a few routine tests, restricted to the most important ones, and on a low frequency can be exercised in industry. But, to obtain reproducibility, it is necessary to run the recoveryand reinforcing operations under strict conditions leading to constant values in water, sulphide and lime. 3. Lastly, it is important to know what happens to the organic matter, the salt and greases as a function of the number of recyclings. Before presenting the results, a final comparison must 65 be made in accordance with the employed procedure. For the eighteenth recycling, we have soaked eight sides, four of which we have limed with the recovered bath of the seventeenth cycle, and four have been limed with a fresh lime liquor. This allowed us to compare two sides through two different liming operations, having alternated left and right side in the two different liming baths. Control of the Unhairing and Liming Effect Visual observations and also those made with a magnifying glass on the pelt surface, after liming, wet-blue and crust, revealed that unhairing was constant over the series of 8 recyclings. At the end of the series, histological preparations have been analysed under the microscope and no difference has been found between the cleanness of the grain layers prepared in a recycled or a new lime liquor. The comparison of both microscopic preparations led to another observation, namely a lower swelling of the hides treated with a recycled liquor. For hides treated with fresh liquor, a weight yield of l080/0 was noted and O4"/;: for recycled liming. Ln order to check if this difference has any meaning, we determined some physico-mechanical properties of the leather. Figures 1 and 2 represent the tear strengths which are nicely grouped in a range you will find on a classical production. The same for lastometer values. From those observations made on a pilot-plant scale, we concluded that recycling does not affect the efficiency of unhairing and liming. Reliability of the Process n order to be able to bring to the required level the NazS and lime concentration it was necessary to know their residual concentration and also the volume of the spent liquor. The reliability of the process will be demonstrated if the recovered quantity of bath, lime and NazS are roughly the same between each recycling (Figs. 3, 4 and 5). The recovery of the volume was on average 85':/, (range 80-90'%,). The recovery of NazS was on average 1-45 '%, (range %). This range is small and indicates a fairly constant exhaustion. Lime stays in the spent liquor at %. The conclusion is that the exhaustion of the lime float is reproducible from one Lycle to another. Hence, it will not be necessary to analyse all the spent liquors in order to recycle them. Nevertheless, it will be necessary to know very well the process that will be used and for these reasons, a number of verifications must be carried out on many conventional batches before carrying out recycling. f this is checked and based on an average absorption percentage, the controls may be sufficient if carried out one or two times a week. ncidentally, if you allow me to come back to what has been said before on the rate of exhaustion, we stated if nothing is recovered 507; of NnzS and 60'x of lime is put into the effluent. We know very well that the NazS concentration must be above a certain level to have the desired reducing power and 2-3 % is a good lower limit. n the case of lime, the process is different and in industry, 3-4 YL, if not more, is commonly used. We have been working with 2% lime

4 --,,.. "... Kg/mm Tear strength..... ^.... -Y... % Residual Na2S Lime Liquor X x n x n Y o * * n x n a... x " X ~......_... ~ - ~.... ~. X.... n *. * X i" FGURE 1 cycles 4 4 mm Lastometer Extension x-.-ic-.ii..il~ii... n... R X x x x ~ x x 0...,_......#.? _... % Residual Lime Lime Liquor --..._... a..._._ x-n.-a...,1.x-*-- - x x x x x.,..k..e.*-..*.... * i..... b A % flesidual Lime Liquor volume 4 % Residual Nitrogen Lime Liquor A, ~- ~ X. f -n.,. Y x -* n X - K *' : Y' ti,,,**ck...--e x x _*._%..C. *-*-* X i..'. * without observing any difference on the leather quality. Hence, from a 2 lime input, we recovered 1.2 %. Lime in excess produces sludge for nothing except unjustified loads on your effluent. 66 Evolution of Other Components We were interested to follow the development of the spent liquor and its ratio of organic soluble matter. The most important factor is the amount of solubilised

5 keratin and nitrogen determination is a simple tool to control build-up of organic matter as a function of the number of recyclings. Figure 6 reviews the obtained figures. n the early cycles, there is a rapid increase in nitrogen until the seventh recycling, after which the curve flattens to a steady concentration in nitrogen content. This phenomena is rather surprising since we expected a linear increase of g/litre between each recycling. We have tried to explain the experimental figures in this way. The residual volume of the spent liquor is between 80 to 85 of the initial bath. On each recycling, this loss (20-15 %) must be compensated for by adding fresh water which means a dilution of the concentration of dissolved nitrogen in the liquor. n the reconstituted liquor, a fresh quantity of keratin will be dissolved to be added to the one already present but corresponding only to 80% of the really dissolved quantity. f we put: A: the concentration of dissolved nitrogen per cycle in g/litre ; n: the number of cycles; V reconstituted d: the dilution ratio: V residual Y: the concentration of N at a given cycle. Then, the following type of equation can be written: Yi =A Yl A A Y2= - +A= +A= [1+d] d d d This leads to a sum of n in terms of a geometric progression, which can be expressed as : y= A &-1.. x --- &- rl-l This equation cannot be directly related to practice as the recovered volumes are not exactly the same for each recycling which means that putting tannery operation in equation is still Utopian. But this theoretical approach is not completely nonsense, as it allows one to prove that a pseudo-stabilisation of the nitrogen content must take place and further that it is possible to predict the value of nitrogen at stabilisation if the average dilution and concentration of A is known. For example, if A=3 g/litre and d= 1.3 Y, will stabilise at 13 g/litre. n our experiment at the last recycling, we obtained g/litre, which is a fair result compared with theory. f Yn becomes very important (for instance, for a low dilution ratio), we can expect to run into difficulties as the liquor will become viscous. Therefore, it is possible from the start to get the system under control by applying the adapted dilution. The fact that the skins contain liquid overconcentrated in proteins during liming, one may fear an important release of organic matter upon deliming. n order to check this point, we determined the nitrogen content in the wash water, in the deliming float, bating float, wash water after bating and on the pickle float. Figure 7 gives the nitrogen contents during these operations. None of the analyses gave rise to significant variations in N content over 18 cycles. n fact only strongly degraded t nitrogen 9/1 Q t 8 t ' 7 x..._?... x x.....n K....!..._. x...7x 6 R 4...x...x... x x ' x. - x delime 3 2 -,,. K...* X.. Y x wash water 6 a- -5. :-! - K -. y.k 1 pickle float 67 -a bating proteins can be absorbed into the pelt: that is a question of molecule size. Moreover, this type of protein is only present in a small portion amongst the other keratin residues and their amount does not change during recycling. Some other factors must be checked. The liberation of chloride is very limited and there again the dilution effect upon recycling can be recorded. The same happens to the grease contents. As inorganic salt concentration remains constant from cycle to cycle, only the organic matter can influence the density of the spent bath. But as this content becomes stable after the sixth to seventh cycles, the density must be steady as well. This is shown in Figure 8. f density t ph The ph value is reduced by 0.2 to 0.3 due probably to the buffer effect of some NHs salts but the ph stabilises rapidly at Finally, we conducted a bacteriological control on the first, on intermediate and the final batches and the results are as follows: the first bath, colonies/ml; intermediate (cycle O), 9000 colonies/ml; final ( 8 cycles), 9000 colonies/ml. f we note that a period of two months separated the first and final operation no bacterial development has taken place.

6 Nevertheless, we agree that these observations have been made on small volumes (70-90 litres) and it is doubtless that these determinations should be repeated when the operation is carried out with industrial batches. Economics To complete what has been explained, a comparative balance has been drawn concerning water consumption and chemicals through a conventional and recycling system. f we count NazS, lff/kg; lime, 0.35 FF/kg, then we economise per ton of hides approximately FF/t or for a 250 day input per year, 4460 FF/year. To this must be added the reduction of pollution taxes. For France, that would be 760 FF/ton year, making a total saving of FF. A tannery with an input of 10 t/day will economise 52,200 FF/year only on his liming operation. From this amount must be subtracted the investments necessary to operate this technique. At this moment, we are testing such a system on a large scale in three tanneries. 68

Tannery Wastewater Treatment

Tannery Wastewater Treatment Tanning means converting animal skin in to leather. Oldest industry in India. This wastewater is characterized by strong colour, high BOD, high ph, high TDS. Manufacturing