Industrial Wastes
TANNING INDUSTRY ROBERT SUTHERLAND, A. C. Lawrence Leather Co.,Peabody, Mass.
b b The numerous metbode employed in leather processing cam b e divided into three basic groupvegetable, minwal, and sbeepekin tanning. The compositions of waates produced by plants using a n y one of these procedures are reasonably similar. There is no standard metbod of t r e a t m e n t and disposal in the industry because of wide differenin local antipollution regulations as well as in types of proceas. Better utilimtion of bypmduct materials is possible and desirable. Bemuse of the relatively bigb solids wbioh cbaraotsriu, m a n y tannery wastes, some form of precipitation and primary sedimentation is included in nearly all disposal systems, and semndary bioIegiml treatment is seldom necessary to meet existing requirements. Pendins s t a t e and federal le&.lation w i l l increase the importance of this problem.
T.
HE treatment and disposal of tannery wastes is a problem which hns been incnvvling in importpnce during the last half
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mtury. In its early day8 the induntry was made up of a large number of d l plants, and, in most cwea, the volumes of waste produced were run into the nearest stream or body of water with little danger of pollution. L a t h e r msnufsctwing today is wncentrated in the hands of relatively few lprser units, and dispossl of wwte k a neoesssry factor in current operations or is likely to be in the not-todistant future. A census supplied by the Tanners’ Counail of America (Y) lhta 7569 tanningestablishments in 1869 employing 35,243 workers, as compared with 477 plants in 1944 employing 41,800 men. In addition, many tanneries are now situated in cities or larger communities where other induatries wntribute their wastea to the local dsposal problem. TANNlNG PROCESSES
The individual wastes from each step in tanning procedure are numerous and varied, depending on the type of leather produced and the process employed. The latter can perbapa best be divided into three basic groups, in esch of which the types of wsstes are reasonahly similar from plant to plant: vegetable tanning ( d e and light leather), chrome and other mineral tsnning (light leather), and sheepskias and shearlings (all tannages) Vegetable tanning is employed in the manufaoture of sole, belting, upholstery, hag and case, and novelty leathers. It is aLS0 used for shoe upper and lining leathers to some extent, but this is usually handled in plants where the majority of the tanning is done with chrome, and where the character of the waste is determined by the latter. Figure 1is a typical proceas flow sheet of a vegetable tanning procesa. Hides and skins, received from suppliers mainly in the green salted wndition, are limb. washed free of dried blood, dirt, and surface salt. They are then soaked far 12 to 24 hours in water to remove salt and rehydrate the stock, after which surplus flesh is removed by muahhe. The hides or skina are now ‘limed’’ to loosen hair and e p i d e r d layer, and to produce the plumpidg
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necessary for penetration of tanning and other materials in subsequent operations. Liming is accomplished by immersion of the stock for 5 to 10 days in a saturated c&um hydroxide solution in which there is considerable excess lime in suspension. Most lime liquors also contain an accelerator to increase the process rate. Sodium sulfide and sodium sulfhydrate are most frequently used for this purpose, but occasionally other alkaline reducing agents or amines are employed. The bides are next unbsired and beamed mechanically to remove as much keratinous msterial ae poesible from the grain. The stock-is then washed in running water and proceeds to the bating operation, which involves immersion for ashort time in a solution of proteolytic enzymes and an acid salt such as ammonium sulfate. ’ The purpose of this step is partially to delime the skins, especially the outer surfaces, and to hydrolyse and remove other undesired proteins not dissolved during soaking and liming. The hides are then tanned in solutions of vegetable tanning materials for relatively extended periods in a series of vats containing rockers to impart slow motion to the stock. The bides are first treated with a weak or tail liquor and then with increaeing concentrationsuntil the cycle is complete. All or psrt of the used tail liquor is discarded daily. After therocker yard tmning, the stock is treated with h a y concentrated tannin solutions for maximum take-up and is then dried. The tanning process alone may require from 6 days to 3 months, depending on the Size of hide and the type of leather required. In the case of heavy leather the tanned stock ia bleached by successive immersions in weak alkali, acid, and water to improve its wlor and remove excees tannin from the surfaces. It is then loaded by milling in drums with oils, Salts, and sugar (to produce the desired temper and weight) and processed through a number of finishing operations, including polishing and rolling. Lieht vegetable leathers are handled in the same basic manner as sole leather through tanning, with variations in timeofprocming and mounts of materials employed. After tanning, the hides or skins’are frequently split to the desired tbickness on a machine, washed, dyed in a dnun, and fat-liquored with water emulsions of any of a number of oils. Such leathers are usuallly coated with pigmented hishes, and are subjected to such mechanical operations as softening, glaaing, ironing, and plating. Chrome tanning is the,method employed to produce the majority of light leathers for shoe uppers and linings, and for garment leathers used in the manufacture of coats and gloves. Figure 2 is a typical Bow sheet of a chrome tanning process. T h e earlier steps in the processing of chrome leather me basically similar to those employed,in manufacturingheavy vegetable leather, but there are a few differences worthy of note. A portion of the raw stock is made up of sun-dried skius which require more prolonged soaking. The lime liquors are usually “sharpendd” with a larger concentration of sodium sulfide; this results in a shorter liming period. Also the bating of chrome leather is more extended, so that a greater degree of deliming is effected. After bating, the skins are pickled in sulfuric acid and sodium chloride solution, usually in vats for 10 t o 16 hour8 (Figure 3). The stock then tanned in a drum (Figure 4) with bwic chromium sulfate and salt solution, . for several hours or overnight.
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May 1947
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Occasionally thia process is carried ouC in vats, and 2 or 3 days are then required. Some producers empko a two-bath method which involves drumming the stock with sodium bichromate and then a redwing agent, such as sodium thiosulfate. In any, of these method@alkali, such as sodium carbonate or bicarbonate, is added toward the end of the pxowes to effect a combinsr tion of chrome salt with skin protein. Othermetallicsalts, such as alumhum or zirconium sulfate, are sometimes employed in tanning, but theprinciples involved are similar. The tannedakinsarefrequently held over for a day or more to permit further c o m b i n a t i o n of chrome, and they are then washed with large volumes of water. Additional alkali i s added to the drum to raise the pH of the stock, and the l a t t e r is a g a i n washed. The coloring, fat liquoring, drying, and finishing which follow are carried out ementially as on vegetable tanned leather. ' Figure 1. Flow Sheet of the The processing of Vegetable Tanning Process SheeDskihs and s h e a ; l i i is not markedly different from the procedures already described. No liming or unhsiring is required; shearlings retain their original wool in the finished product, and sheepskin wool is removed a t time of slaughter. Both types of leather may repuire d e g r e e ing, however. Sheepskins are ordmarily degreaaed in a drum with petroleum solvent and detergent, which is subsequently removed by wsahing with salt solution. Some sheepskins are processed into chamois leather, which involves impregnating the skins with fish oil, oxidismg this oil by hanging the skinsin dryins lofts, and then removing the excess uncombined oil by washing with an alkali such as soda ash.
Table 1. Bmic Types of Tannery Wastes
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Llohble ended Lldida Bolids h i @Vola- Ash B.O.D. P.P.ad. P.P.M. P.P.ik tile. % %' P.P.M.' pH 16.W 13,510 1490 1.23 . 0.27 1200 6.6
Bource Of Waste Soak Fleshing .... Lime .26.W Hair .... .... Wash. all types Bate 4,410 Pickle 81.203 Vegetable tan l8ACQ Chrqme tan 70;m Bleaah .... Color and fat 1iq"Or 2.460 Degroasa ChDlOis
.... ....
....
i8.w
.... i's&
69:W 17.110 74,810
....
2,010
.... ....
vi& .. iio
1240 1290 1990
..
480
.. ..
i:i
..
i:i
..
aiio
a:az
o:ia
iio
..
764
0.59 6.53 1.50 0.34 1.58. 8.10
..
..
0.03
.0,.18
..
..
.. ..
.. 6 : ~
5500 818
2.4 6.0 a.3
172
3.9
..
'
ii:e
..
..
..
.. ..
soluble calcium sal&. Hair, like fleshings, is a sonrce of contamination only when inadvertently allowed to mix with other wastes. Wash waters are generally so reletively large in volume that they preamt no problem of disposal hut merely seme to dilute the other wastes. Bate liquors vary widely in composition according to the proceaa used, running from rather low so!id@ and B.O.D. in heavy leather work to moderately high values in upper lesther production. Pickle liquors contain appreciablequantities of dissolved salt and acid, and B.O.D. values depend on the frequency of mering. The spent liquors from vegetable tanning are fairly large in volume, contain much organic material in solution, and me the major contributor to the B.O.D. ae well as the color of heavy leather plant effluents. Exhaust chromeliquors areevenhigherin dissolved solids, consisting c h i d y of inorganic salts and acid, but they are much d e s m volume and lower in B.O.D. Bleach liquors are much like spent tan, , although lower in dissolved solids and total volume. The wastes from coloring and fat-liquoring upper leather do not usually contain objectionable quantities of any substances, even oil or dye, and diution by ihe large amount of water used in washing during these operations d e s suck duents wmparatively innoxious. The spent liquors from degreasing contain solvent and grease which should be removed prior to discharge; this is likewise true of the oily w a s h from chamois taming. COBRECTIVB MEASURES
T h e corrective measures taken for d m a l of waste by any given tan-
DESCRIPTION OF WASTES
The basic types of waste resulting from thetannery procedures mentioned are listed in Table I with analytical data pertaining to the more important item. Soak liquors are moderately high in dissolved solids consisting of salt and some organic matter. The S-day biochemical oxygen demand (B.O.D.) varies widely, depending on the degree of prior washing and length of soaking time. Fleshings are usually prevented from entering the sewerage system, but, if permitted to do so, they contribute to the over-all organic content of the tanner waste. Lime l i u o r s are high in dissolved wlids, mpended solids, alkalinity, and B.O.D. Although not true of the example shown in Table I, they often contain as much as 6-8% of in-
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BUS
Tots1
Sheet of *' Chrome Fiyn l%W Tannine P m m
nerysredictated by severalconsidere tions. Location is a prime factor. Many leather plants nm their &uents into municipal sewage system, nnd where the volumes of such wastes are not relatively large, no prior treatment may be necessary. Most of the tanneries which operate in this man. ner are situated on the East Coaet, where waste may be run into tidewaters and requirements are far lees stringeat than is the case when inland stmfms and lakes are used for disposal. In the latter instanoe
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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rnniisty 'of tanking the spent liquor and using it in combination with fresh liquor for subsequent processing. A second method involves dkaline precipitation uf chromium hydroxide, filter pressing, and re-solution in sulfuric acid. Both of the-e procedures n-ere eiuployed by ionie tanners i n tiii. count1 during the chromium bhortage of tlie h h t n-nr, with improvement in economy and reduction i n u-azte discharged. Large volumes ( ~ fs1)ent vegetable tanning liquor are sewered daily by heavy leather plants and, t o a lesser extent, by those producing light leathers. Much effort 1ia.j been expended t o x a r d the development of ii huitahle method for recovering tlie nppreciable amounts of tannin theye liquors contain, thus far n-ith no real buccess. A sulutiuu tu this problem \vouldsimplify imnieasurahl~then ahte disposal problem of all vegetable tannerieh. Figure 3.
Bating and Piclilirip \-ats SEGREGATlON
the ampunt of waste from a tannery in relation t o tlie volume of t h e receiving body of Tvater may determine the required trentrnent. A second important consideration concerns the antipollution regulations which exist in t h e area where the plant is located. -1 number of plants are currently operating on the banks of rivers under no legal compulsion to treat their wastes before discharging them, and still others are required merely to reduce t o a minimum the suspended solids in their effluents. Only relatively few tanneries have t o contend with strict requirements in regard t o dissolved solids and B.O.D. content. Pending legislation is expected t o materialize in t h e next few years which will require more thorough waste treatment in a majority of industrial plants. Government regulation xi11 probably be established by state laws, with federal cooperation in t h e case of rivers which flow through more than one state. Federal subsidization is likely and should help t o speed the program toward completion. UTILIZATION OF BY-PRODUCTS
Any program of waste disposal should include means of maximum utilization of waste materials as by-products. A number of types of waste are handled by all tanners in such a manner t h a t it marketable or usable product is obtained (Table I), and stili others have potential value, currently realized Only in isolated cases, if at all. As mentioned previously, hair and fleshingh are prevented froiii mixing wit,h the liquid wastes as muck as po>*il)lc,and both of these material; have a ready market in normal times. E:xhaust liquors from degreasing drums are tanked to effect removal of water fi.om grease and solvent, and tlie latter mixture ib then separated tJy distillation. This grease ha? definite value in the . w i l i induptry, and the solvent must be re-used to niaintain net degreasing co,t at a reasonable level. If the vycle is operated effriently, no appreciable amount of contamination should enter thr, sewer from thi, sourre. For similar reasow t h e alkaline liquor.: from rharnois tanning a.re aridified to rerovet' nioellon degras, or sod oil. The wspended matter in spent linie liquor- i. usually separatecl by settling them, either alone or mixed with the other plant wastea; this material is occasionally sold as fertilizer, frequentlJfor no more than the c o i t of hauling. Pickle liquors contain no recnverable materials of an>- value, but, with careful cliemirnl control they may he re-wed for extended periods. This reducer; to a minimum the amount which is selvwed. Most exhaust chrome liquors possess appreciable amounts of chromium which is recovered by a few tanners. One method
Segregation is advisable, where possible, for thvae wa,tes which do not require treatment under the regulations &sting in the area. Such a practice reduces the volume of inaterial vihich must be handled by the treatment plant and lience the cost of operation. The following table shows distribution of wastes from a typical side leather plant which produces 730.000-1,000,000 gallons of effluent daily :
Tlir \Tastes from the three listed washing procedures are low in suspended solids and usually in dissolved solids as well. If the plant is located where minimum suspended solids is the only requirement, such wastes may be discharged directly t o the receiver, \vith a consequent reduction in load on the treating unit of almost 50%. It is obvious, of course, t h a t such wash waters should be inixed with the treated effluent before final discharge in order t o tlilute the latter further. The first step in actual processing of waste is usually screening tu remove large particles such as hair, fleshings, and shavings. Bur screens are frequently employed for this purpose, with either nianual or mechanical means of clearing the solids from the surt x e of the screen. Even more effective are rotary-type screens with to 2 !p-incJi openings, kept clean by their motion or by \v:ltPl,
