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第17 卷第4 期 皮 革 科 学 与 工 程 Vol117 ,No142007 年8 月L EATHER SCIENCE AND ENGINEERING Aug1 2007Artical ID :1004 - 7964 (2007) 04 - 0003 - 06Received Date :20072052213 P. Mokrejs :Corresponding author. Phone N°: + 420 57 603 1230 ;Fax N°: + 420 57 603 1563 ;e2mail :mokrejs @f t . utb. c2Recycling Technology for Waste Tanning LiquorsP. Mok rejs1 3, D. J anacova2 ,M. Mladek1 , K. Kolomaz nik2 , F. L angmaier1 ,V . V asek2(1. Tomas B ata University , Faculty of Technology , Department of Protein and Leather , nam.TGM 275 , 762 72 Zlin, The Czech Republic ;2. Tomas B ata University , Faculty ofA p plied Inf ormatics , Institute of Processing Cont rol and A p plied Computer Science ,N ad S t ranemi 4511 , 760 05 Zlin, The Czech Republic)Abstract :Tannery belongs to typical indust rial branches which during t he technological processp roduce considerable quantities of liquid and solid wastes cont ributing to polluting t he environ2ment . The presented work describes dechromation of waste tanning liquor utilising chrome sludgep roduced t hrough enzymatic hydrolysis of chrome2tanned leat her wastes. The principle of dechro2mation consist s in sorption of chrome f rom waste liquor af ter tanning to chrome sludge. Recy2cling technology is characterised by an almost 99 % efficiency , simplicity and low investment re2quirement s.Key words :solid waste ;liquid waste ;chrome sludge ;waste tanning liquor ;chrome recoveryCLC number :X 794 Document code :A鞣制废液的循环技术摘 要:制革属于典型的工业分支,制革工业中会产生大量的污染环境的废液和废固。该论文利用铬鞣革废物的酶法水解用铬污泥除去鞣制废液中的铬。除去铬的原理是鞣后的铬污泥从废液中吸收铬。该循环技术回收率可达99 % ,操作简单、成本低廉。关键词:固体废弃物;液体废弃物;铬污泥;鞣制废液;铬回收1 INTRODUCTIONThe tanning indust ry p rocesses wastes of themeat indust ry , raw hides , and t ransforms t hemt hrough chemical , chemico2p hysical and mechani2cal processes into leather s. During t he technologi2cal process , solid and liquid wastes arise which ,particularly as a consequence of t he cont rover sialchrome t hey contain , burden the environment .Processing one met ric ton of raw hide produces ap2prox. 200 kg finished leat her . About 30 , 000 kgwastewaters arise through processing and moret han 70 % chrome passes into liquid and solidwastes [1 ] .In classic tanning , approx. 40 % chrome (t hestarting charge of 15 - 17 kg Cr / t ) remains inleat her , 26 % - 30 % chrome passes into solidwastes and 30 % - 34 % chrome remains in liquidwastes. In t he tanning of hides using technologicalprocedures ensuring high removal of chrome com2pounds by white hide , 54 % - 57 % chrome remains© 1994-2008 China Academic Journal Electronic Publishing House. All rights reserved. leat her , 31 % - 38 % chrome passes into solidwastes and 5 % - 15 % remains in liquid wastes [2 ] .For modifying t he tanning process , organic acids ,e. g. are used , which cont ribute to improving theremoval of chrome ions.1. 1 Liquid wasteManufacture of chrome2tanned leat hers pro2duces 115 - 2 times quantities of wastewater s t hanmanufact ure of vegetable2tanned leat her s [3 ] . Los2ses of chrome compounds in waste tanning liquor smay be prevented in several ways. The simplestapproach is the direct recycling method consistingin re2using exhausted liquor in t he tanning process.The main shortcoming of t his method is a negativeeffect of salt s and other contaminating substanceson quality of finished leather . Af ter being recycledseveral times , wastewater has to be discharged intosewage [4 ] . Anot her potential met hod is indirectrecycling , in which chrome is obtained f rom ex2hausted sludge t hrough precipitation using a suit2able alkaline agent , e. g. sodium hydroxide , sodi2um carbonate , sodium hydrogen carbonate , mag2nesium oxide , calcium sulp hate , calcium hydroxide[5 ] . A f urt her met hod consist s in employing elabo2rate techniques , e. g. elect ro2dialysis , membraneseparation , ion exchangers , wit h which chromemay be separated f rom ot her salt s contained in ex2hausted sludge. Chrome thus obtained is character2ised by much higher p urity than that obtainedt hrough indirect recycling. Disadvantages are ahigh cost and limited po ssibilities of applying t hesedemanding technologies in tanning factories [ 4 ] .Unutilised chrome in liquid wastes , apartf rom significant economic losses for tanneries , al soposes great hazard for t he environment and forman. The most stable and most important oxida2tion state is Cr ( III) . Compounds of Cr (VI) occurin t he form of chromates and dichromates and ex2hibit quite st rong oxidative properties [6 ] .Chrome is found in liquid tanning wastes in it st rivalent form , there exist s , however , potentialdanger of it s oxidising to hexavalent compounds ,particularly when t reating groundwater s to drink2ing waters. Hexavalent compounds , when com2bined wit h calcium or magnesium ions , are st rong2ly carcinogenic[7 , 8 ] .An extensive research exploring negativeeffect s of tanning wastewaters f rom local tannerieson t he adjacent ecosystem was conducted in Indi2a[9 ] . Highest level s of chrome contained in soilwere found in t he vicinity of tanneries , at approx.60 g/ kg. Highest chrome accumulation was ob2served in cauliflower stalks (72 mg/ kg) in locali2ties near tanneries. In st udies investigating Cr ac2cumulation in fish organs it was found t hat mostchrome accumulates in t he liver (approx. 22 mg/kg) and least in muscles (approx. 1 mg/ kg) . Thepo ssible t ransfer of hazardous Cr doses into t hefood chain and consequently to human beings ist hus quite obvious.112 Sol id wasteTannery processing al so produces a considera2ble quantity of solid chrome2tanned wastes whichpresent a significant problem due to t heir chromecontent . The problem of solid wastes has beensolved so far by land filling which , of course , be2side ever increasing land filling co st s al so bringst he danger of chrome escaping into t he ecosystem.Due to t heir containing a sizeable proportion ofprotein material , solid tanned wastes started to beprocessed. Enzymatic hydrolysis has being em2ployed to considerable extent in latest year s , wit hit s particular advantages being mild reaction condi2tions and economic cost2effectiveness [10 , 11 ] . Themain product of enzymatic hydrolysis is collagenhydrolysate , whose application possibilities arebroad , e. g. as feed additive , nit rogenous fertili2ser , raw material for preparing adhesives , corro2sion inhibitor s , etc. The remaining waste productaf ter enzymatic hydrolysis (chrome sludge) has notyet found lucrative application. It s employment ,due to chrome content , has not been much adoptedfor preparing combined tanning bat hs.Subject of the presented work is potential use4 皮革科学与工程 第17 卷© 1994-2008 China Academic Journal Electronic Publishing House. All rights reserved. t he waste product af ter enzymatic hydrolysis ofchrome2tanned solid wastes (chrome sludge) for i2solating chrome compounds f rom waste tanningliquor s.2 MATERIALS AND METHODSSolid was determined according to ISO 4684[12 ] , ash according to ISO 4047 : 1998[13 ] , nit rogenaccording to ISO 5397 : 1984 [14 ] , Cr2 O3 accordingto ISO 5398 [ 15 ] , Cr according to ISO 9174 : 1998[16 ] and Mg according to ASTM D511203 [ 17 ] .Chrome sludge is a waste product of enzymatichydrolysis of chrome2tanned solid waste and it scomposition is given in Tab. 1. Typical composi2tion of chrome2tanned solid waste is given in Tab.2.Tab11 Composition of chrome sludge1)表1 铬污泥成份Parameter ValueTS/ % 1614Ash/ % 3410N / % 114Cr2O3 / % 1215MgO / % 1115 1) based on total solidsTab12 Composition of chrome2tanned solid waste1)表2 铬鞣革固体废弃物成份Parameter ValueTS/ % 6911Ash/ % 1019N/ % 2011Cr2O3 / % 415 1) based on total solidsTab13 Composition of waste tanning liquor表3 鞣制废液成份Parameter ValueTS/ % 718Ash/ %1) 8713N/ %1) 018Cr/ (mg/ kg) 1 ,769Mg/ (mg/ kg) 693p H 412colour blue2green 1) based on total solidsWaste tanning liquor was obtained f rom tan2ning of cowhides and it s composition is presentedin Tab. 3.The experimental part was divided into twopart s. The fir st part contained five selected massratio s of chrome sludge ∶ waste liquor and threevarious dechromation times , see Table 4. Practicaldechromation was performed under laboratory con2ditions by stirring chrome sludge with waste tan2ning liquor at room temperat ure in accordance wit ht he scheme shown in Fig. 1. Af ter dechromation ,t he heterogeneous mixt ure was cent rif uged. A partof supernatant was analysed for Cr content . Theinvestigated object in view was quantity of residualchrome in waste tanning liquor af ter dechromation.The second part of test s followed up result s ofwaste liquor dechromation efficiency in t he first ex2perimental part . The mass ratio of chrome sludge∶waste liquor producing greatest dechromation ef2ficiency was selected and a series of test s per2formed to t he p urpo se of st udying dechromation re2action kinetics.Fig1 1 Scheme of dechromation of waste tanning liquor图1 鞣制废液的脱铬流程3 RESULTSTest s and dechromation result s at variousmass ratios of chrome sludge ∶waste liquor and va2rious dechromation times are shown in Tab14.Tab15 t hen shows dechromation result s at mass ra2tio of chrome sludge ∶waste liquor = 1 ∶1 and va2rious dechromation times. Each test was performedt hreefold and arit hmetic mean calculated , standarddeviation ranged wit hin ±5 %.第4 期 P. Mokrejs ,et al :Recycling Technology for Waste Tanning Liquors 5© 1994-2008 China Academic Journal Electronic Publishing House. All rights reserved. Dechromation results of waste tanning liquor at mass ratios of Cr sludge/ waste liq. and dechromation times表4 铬污泥与废液不同质量比以及不同脱铬时间条件下鞣制废液的脱铬结果TestNo.Batch( g)sludge :liquort/ minIn2processpHCr liq. /( mg/ kg)Eff iciency/ %1 7114 ∶50 30 6134 - 7108 807 54142 90 190 89123 180 116 93154 20183 ∶50 30 6195 - 7148 146 91175 90 58 96176 180 33 98117 35171 ∶50 30 7121 - 7168 53 97108 90 33 98119 180 36 981010 50 ∶50 30 7130 - 7190 24 981711 90 23 981712 180 17 991013 70 ∶50 30 7148 - 8102 41 971714 90 32 981215 180 79 9516 1) Starting content of chrome in waste tanning liquor = 1769 mg/ kgTab1 5 Dechromation results of waste tanning liquor andtimes1)表5 铬污泥与废液质量比为1 ∶1 不同脱铬时间条件下鞣制废液的脱铬结果TestNo.t/ minIn processpHCr liq./ ( mg/ kg)Eff iciency/ %1 4 7120 - 7133 438 75122 8 7127 - 7145 351 80123 12 7151 - 7189 299 83114 16 7176 - 7194 230 87105 20 7180 - 7191 206 88146 24 7181 - 7194 200 88177 28 7181 - 7196 139 92118 32 7182 - 7199 43 97169 36 7188 - 8103 32 981210 40 7190 - 8104 19 981911 60 7186 - 8106 28 981412 80 7187 - 8107 22 981813 100 7180 - 8111 35 9810 1) Starting content of chrome in waste tanning liquor =1769 mg/ kg ;mass ratio of chrome sludge ∶waste liquor = 1∶1Figure 2 shows column plot s representing re2sidual chrome content in waste tanning liquor af tert he dechromation procedure at various chromesludge ∶waste liquor ratios and various dechroma2tion times. With a mass ratio of 1 ∶7 af ter 30 mindechromation , a drop of chrome in liquor exceeding50 % was found , af ter 180 min the dechromationefficiency was greatest . Wit h a ratio of 1 ∶214 ,t he tendency of chrome content in liquor to de2crease wit h time was at it s greatest , residualchrome content in liquor coming down to 3314 ppmwhich represent s dechromation efficiency exceeding98 %. Wit h a ratio of 1 ∶114 t he tendency ofchrome decrease is similar , when t he greatest dropof chrome content in liquor was already recordedaf ter 90 min of dechromation ( to 3312 mg/ kg) .Wit h a ratio of 1 ∶1 , lowest level s of residualchrome in liquor were already at tained in 30 min ofdechromation app rox. 24 mg/ kg , representing al2most 99 % dechromation efficiency. A f urt her in2crease in dechromation times produced but negli2gibly decreased chrome content in liquor . Wit h aratio of 114 ∶1 , t he greatest drop of chrome con2tent in liquor was recorded af ter 90 min dechroma2tion. Fig13 shows dechromation kinetics of wastetanning liquor at mass ratio of chrome sludge ∶waste liquor = 1 ∶1 and various dechromationtimes. From result s it is obvious t hat af ter 40 min6 皮革科学与工程 第17 卷© 1994-2008 China Academic Journal Electronic Publishing House. All rights reserved. chrome content in waste tanning liq2uor decreased to approx. 20 mg/ kg , which com2pared wit h starting content of chrome in waste tan2ning liquor ( 1769 mg/ kg) signifies almost 99 %chrome removal efficiency. Further increases indechromation time did not show in markedly in2creased chrome removal efficiency , and values ofresidual chrome content in slurry fluct uated be2tween 20 and 35 mg/ kg.4 DISCUSSIONDuring past decades , a t ransfer occurred of anotable part of tanning plant s f rom economicallyadvanced west European count ries to developingcount ries and to count ries of t he Asian continent .In latest year s , however , t he environmental loadproduced by t hese plant s became unsustainable andintensive negotiations on resolving t he mat ter gotunder way. When rest ructuring existing and con2st ructing new tanning plant s , issues coming partic2ularly under consideration are solid and liquidwastes containing chrome. Contamination ofgroundwater s wit h liquid wastes containing chromeis obvious. Release of chrome f rom solid wastes第4 期 P. Mokrejs ,et al :Recycling Technology for Waste Tanning Liquors 7© 1994-2008 China Academic Journal Electronic Publishing House. All rights reserved. proceed particularly t hrough elution by rainwith great hazard of groundwater contamination.The danger of Cr ( III) oxidising to Cr (VI) was al2ready mentioned in t he fir st part of this cont ribu2tion.Test result s showed t he solid waste product ,chrome sludge , may be successf ully employed to i2solate chrome f rom waste tanning liquors , and t hatwith an efficiency of app rox. 99 %. The advanta2ges of recycling technology may primarily comprisesimplicity and low demand for investment cost s ,which make it ideally applicable when recyclingwaste tanning liquor s especially in tanning plant sin t he developing world.Utilisation of chrome2enriched sludge remainsan open issue. A feasible alternative is it s combus2tion and separation of chrome , which may be used ,e. g. in pigment manufacture. A particular compli2cation is presented by accompanying magnesium ,which is undesirable for t hese applications. Isola2ting magnesium f rom chrome sludge will be subjectof a following research. Employment of chrome2enriched sludge for p reparing f resh tanning bathsal so remains an open issue.5 CONCLUSIONRecycling technology developed for removingchrome f rom waste tanning liquors wit h applicationof waste chrome sludge achieves almost 99 % effi2ciency. Dechromed waste liquor represent s a mini2mal load on t he environment . Chrome2enrichedsludge , following isolation of ot her accompanyingelement s (especially magnesium and calcium) com2plicating it s f urther processing , could serve to pre2pare pigment s. Result s of experimental measure2ment s and an elaborated mat hematical model of re2cycling technology will serve to propose an algo2rit hm of enclosed dechromation cycle cont rol whichwill be t he subject of pilot2plant test s.Acknowledgements : The aut hors would like tot hank to The Minist ry of Education of The CzechRep ublic for financial support to t his work execu2ted under MSM Grant No : 7088352102.References :[1 ] Process technology for recovery and recycling of chromi2um f rom leather waste and sludge , BLC , EV5VO542(1994 1996) .[2 ] Ludvik J . Chrome balance in leather processing [ J ] .UNIDO , US/ RAS/ 92/ 120/ 11 - 51 (2000) .[3 ] Mladek M ,et al . Leather indust ry waste t reatment [M] .State Technical Literature Publishing , Prague ( 1971) :261 - 265.[4 ] Rajamani S. A system for recovery and reuse of chromi2um f rom spent tanning liquor using magnesium oxide andsulphuric acid[J ] . UNIDO (2003) .[ 5 ] Covington A D , Sykes R L , Barlow J R , et al. A practi2cal chrome recovery system using magnesium oxide[J ] . JSoc Leather Technol Chem ,1985 , (69) :166 - 174.[6 ]Cotton F , Wilkinson G. Advanced Inorganic Chemist ry( A Comprehensive Text ) [ M ] . Academia , Prague(1973) . 788 - 803.[7 ] Hartford W H. Proceedings Chromium Symposium [J ] .Indust rial Health , Foundation , Pitt sburg (1989) .[8 ]Vijayalakshm R . A Study of the interaction of Cr ( III)complexes and their selective binding with B2DNA. Amolecular modelling approach[J ] . J Biom St ruc Dynam ,2002 (19) :1063 - 1072.[9 ] Chattopadhyay B. The environmental impact of wastechromium of tannery agglomerates in the east Calcuttawetland ecosystem[J ] . J Soc Leather Technol Chem ,2000(84) :94 - 100.[ 10 ] Cabeza L F. Isolation of protein product s f rom chromi2um2containing leather waste using two consecutive en2zymes and purification of final chromium product : Pilotplant studies [ J ] . J Soc Leather Technol Chem , 1999(83) :14 - 19.[11 ] Langmaier F. Product s of enzymatic decomposition ofchrome2tanned leather waste [J ] . J Soc Leather TechnolChem , 1999 (83) :187 - 195.[12 ] ISO 4684 , Determination of volatile matter.[ 13 ] ISO 4047 : 1998 , Determination of sulphated total ashand sulphated water insoluble ash.[14 ] ISO 5397 : 1984 , Determination of nit rogen and hidesubstance.[15 ] ISO 5398 , Determination of chromic oxide.[16 ] ISO 9174 :1998 , Water quality 2 Determination of chro2mium 2 Atomic absorption spect romet ric methods.[17 ]ASTM D511 - 03 , Standard test methods for calciumand magnesium in water.8 皮革科学与工程 第17 卷© 1994-2008 China Academic Journal Electronic Publishing House. All rights reserved.

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