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