Management Of Leather Tanning And Finishing Wastes
-
Upload
nirmala-last -
Category
Business
-
view
4.849 -
download
5
description
Transcript of Management Of Leather Tanning And Finishing Wastes
Management of Leather Tanning and Finishing Wastes
Thomas C. Voice
Michigan State University
Description of Industry
General: conversion of animal skins into leather, with the option of finishing the leather surface
Raw materials– cattle, sheep, pig and horse hides most common– specialty leathers from various other animals
Products– consumer: apparel, shoes, upholstery– industrial: belting, cushioning
State of Industry
steady decline in number of tanneries and amount of leather tanned world-wide
general decrease in profitability of U.S. tanneries specialized tanneries may be profitable most tanneries are privately owned, use older
facilities and are not highly automated
Basic Operationsraw or cured hides
beam house wash and soak hides, remove hair
tanyard stabilize hide with tanning agent
retan adjust the color and mechanical properties
finish adjust the surface characteristics
leather
Beam House
Receive and sort hides– brine cured (most common), green cured, fresh green– curing displaces water with salt, preserves hide– hides packed in excess salt, bactericides
Side and trim– remove parts of hide that do not make good leather– solid waste sold to rendering plant
Weigh and sort
Beam House (cont.)
Soak and wash– hides soaked in vats, drums or hide processors for 8-
20 hours– removes salt, dirt and manure, restores moisture
Fleshing– mechanical removal of excess fat and muscle– may produce solid and liquid waste streams
containing TSS, BOD and oil/grease
Beam House (cont.)
Unhairing– add lime, sodium sulfhydrate, or sodium sulfide– destroys or loosens the hair– loosens the epidermus (hide swells 2x)– removes some soluble proteins – performed in vats, drums or hide processors– hair burn: complete dissolution (most common)– hair save: loosening with mechanical removal– produces approximately 30% of total waste
Beam House (cont.)
Lime splitting– separate hide into grain and split layers (optional)
Tanyard
Deliming and bating– add ammonium sulfate or ammonium chloride and
enzymes to » remove alkaline depilatory chemicals, reduce pH
» reduce swelling
» peptize the fibers
» remove protein degradation products
» softens the hide
– wash to remove deliming and bating chemicals
Tanyard (cont.)
Pickling– add acid and salts to reduce pH and facilitate tanning
Tanning– reaction of tanning agent with collagen fibers to provide
long term stability– chrome tanning: Cr(III), highest quality leather– vegetable tanning: wood extracts, heavy leathers– other agents: alum, zirconium, aldehydes (rare)– wastes include tanning liquor, fibers, rinse water
Tanyard (cont.)
Wringing– remove excess water– yields wet blue stock, stable
Splitting (if not previously split) Shaving
– to produce hide of uniform thickness– shavings major solid waste stream
Retan
Retan– second tanning for specific appearance or mechanical
properties– solutions are generally more dilute
Bleaching– color lightening– sodium bicarbonate, sulfuric acid common for shoes
Retan (cont.)
Coloring– even out color variations– achieve color penetration
Fat liquouring– addition of vegetable, animal or synthetic oils– replace oils lost in tanning– lubricates the hide, improves the feel
Finishing
Setting out - smooths and stretches Drying Conditioning - adjust moisture content Staking - stretches and softens Dry milling - tumbling to soften Buffing - abrasive smoothing of surface Finishing - surface coating Plating - smoothes and bonds coating
Wastewater Sources
Soakingpits
Liming vats
Defleshing &
DehairingDeliming Tanning
Washing &
Finishing
Continuos flow waste waterSoakingwash
Limingwash
Defleshing &dehairingliquor
Deliming wash water
Tanning washwater
Dying &fat-liquorwash
Rawhides
Spent soakliquor
Spent limeliquor
Spent deliming &batingliquor
Spent tanningliquor
Spentfinishingliquor
Intermittent flow waste water
Types Of Spent Liquors
Soak liquor: soluble proteins, dirt, salts, putrefied organics
Lime liquor: lime, proteins, sulfide, fats, solids, high pH, BOD, nitrogen
Bate liquor: BOD, nitrogen Vegetable Tanning liquor: COD, BOD, salts, Chrome Tanning liquor: Cr, COD, BOD, NaCl,
mineral acids
Process Waste Characteristics
BOD Conc. BOD % TSS Conc. Volume %
Soaking 2,200 20 30,000 42
Unhairing 15,500 54 78,000 32
Delime, bateand pickle
4,500 17 35,000 15
Chrome tan 6,500 6 93,000 5
Color andfat liquor
2,000 3 13,000 6
Composition Of Combined Wastes
Paramater Maximum AverageHourly Value
Average flow 2000-3000 m3/dayAlkalinity 700 – 800 mg/L as CaCO3
pH 12 – 13Chloride 10,000 – 12,000 mg/LDissolved solids 22,000 – 24,000 mg/LSuspended solids 3200 – 3400 mg/LCOD 5000 – 7000 mg/LBOD 3000 – 4000 mg/LChromium 2000 – 3000 mg/L
Chromium Mass Balance
Resource Recovery - Washing
counter current rinsing
– produces much less wastewater – capital cost is minimal if hides already being moved
TanningVat
Rinse 1 Rinse 2 Rinse 3rinsewaterin
rinse effluentto waste
leather flow
Resource Recovery - Unhairing
reuse– pump liquor to storage/treatment tank– remove impurities: solids, proteins (necessary?)– re-fortify– reuse
savings from 20x reuse– 20 fold reduction in effluent sulfides– 7 fold reduction in effluent lime and protein– 5-20 fold reduction in water use
Resource Recovery - Tanning
Reuse - method 1– segregation– purification (optional): remove solids, oil and grease– refortification– reuse: 20x or more tested
Benefits– typically 2/3 of Cr taken up, 1/3 Cr cost savings– substantial reduction in Cr in waste
Resource Recovery - Tanning (cont.)
Reuse - method 2– use spent Cr liquor as make up for pickle liquor– pretanning improves speed of tanning, less salt use
Reuse - method 3– use spent Cr liquor in retan
Reduction– combined pickle/tan with Cr reuse– 20 to 25% Cr use reduction
Resource Recovery - Tanning (cont.)
Recovery– precipitation/acid dissolution of wastewater– ion-exchange– acid dissolution of solids– incineration, recovery of Cr (IV) from ash
Resource Recovery - Proteins
Process
– Acid precipitation
– Hydrolysis
– Ultrafiltration
– Centrifugation Benefits
– high value product recovery
Resource Recovery - Grease
Process– Separation
– Rendering Benefits
– reduce COD, BOD
– low value product
Resource Recovery - Sulfide
Process– Acidification
– Direct recycling to spent lime liquor Benefits
– reduced toxicity of waste
Resource Recovery - Nutrients
Process– collect, dry and granulate solid wastes
– mix with urea (tankage), phosphate, other ingredients
– distribute as fertilizer Benefits
– recover nitrogen,
– avoid discharge to surface water
Waste Treatment - Scheme
Screening Coagulation/Flocculation Sedimentation
BiologicalTreatment
Influent Effluent
Solids Dewatering
Untanned Solids
LandfillLand applicationReprocessing
Sedimentation
TannedSolids
rendering
SolventVapors
Solvent recoveryor vapor treatment
Waste Treatment - Goals
Screening– remove hair, fibers and large solids
Coagulation/Flocculation– add to coagulant for suspended solids
– raise pH, precipitate Cr Sedimentation
– remove 90% of suspended solids
– remove 40% of BOD
Waste Treatment - Goals (cont.)
Biological treatment– 85 to 95% BOD reduction
– remove color, tannins
– activated sludge, trickling filters, lagoons Solids dewatering
– produce a manageable sludge Vapor treatment or recovery
– primarily solvent vapors from finishing
Summary
Leather tanning produces both high strength and hazardous wastes that must be properly managed
Understanding processes and material flow is essential for effective waste management
Numerous resource recovery options exist Conventional waste treatment processes are available and
well understood Economic aspects of environmental compliance represents
the major challenge