Waste Treatment in the Process Industries - Chapter 12 pps

The rubber processing industry is divided into 11 subcategories based on raw waste loadsas a function of production levels, presence of the same or similar toxic pollutants resulting fro

SIC 2822: Synthetic Rubber Manufacturing (vulcanizable elastomers);

SIC 3011: Tire and Inner Tube Manufacturing;

SIC 3021: Rubber Footwear;

SIC 3031: Reclaimed Rubber;

SIC 3041: Rubber Hose and Belting;

SIC 3069: Fabricated Rubber Products, Not Elsewhere Classified; and

SIC 3293: Rubber Gaskets, Packing, and Sealing Devices

Approximately 1650 plants exist in the United States and have production ranges from1.6  103kkg/year (3.5  106lb/year) to 3.7  108kkg/year (8.2  108lb/year) Table 1

the number and types of dischargers Table 2presents a subcategory profile of best practicalcontrol technology currently available (BPT) regulations (daily maximum and 30-day averages)[2] The effluent limitations are shown as kilogram of pollutants per 1000 kg of raw materialprocessed (kg/kkg)

545presents a summary of the rubber processing industry regarding the number of subcategories and

The rubber processing industry is divided into 11 subcategories based on raw waste loads

as a function of production levels, presence of the same or similar toxic pollutants resulting fromsimilar manufacturing operations, the nature of the wastewater discharges, frequency andvolume of discharges, and whether the discharge is composed of contact or noncontactwastewater Other primary considerations are treatment facilities and plant size, age, andlocation The 11 subcategories are listed below A brief description of each subcategory follows

Subcategory 1: Tire and Inner Tube Manufacturing;

Subcategory 2: Emulsion Crumb Rubber Production;

Subcategory 3: Solution Crumb Rubber Production;

Subcategory 4: Latex Rubber Production;

Subcategory 5: Small-Sized General Molding, Extruding, and Fabricating Rubber

Subcategory 8: Wet Digestion Reclaimed Rubber;

Subcategory 9: Pan, Dry Digestion, and Mechanical Reclaimed Rubber;

Subcategory 10: Latex-Dipped, Latex-Extruded, and Latex Molded Goods;

Subcategory 11: Latex Foam

Subcategory 1 Tire and Inner Tube Manufacturing

The production of tires and inner tubes involves three general steps: mixing and preliminaryforming of the raw materials, formation of individual parts of the product, and constructing andcuring the final product In total, 73 plants use these general steps to produce tires in the UnitedStates

The initial step in tire construction is the preparation or compounding of the raw materials.The basic raw materials for the tire industry include synthetic and natural rubber, reinforcingagents, fillers, extenders, antitack agents, curing and accelerator agents, antioxidants, andpigments The fillers, extenders, reinforcing agents, pigments, and antioxidant agents are addedand mixed into the raw rubber stock This stock is nonreactive and can be stored for later use.When curing and accelerator agents are added, the mixer becomes reactive, which means it has ashort shelf-life and must be used immediately

Table 1 Industry Summary

Industry: Rubber processing

Total number of subcategories: 11

Number of subcategories studied: 3a

Number of dischargers in industry:

† Direct: 1054

† Indirect: 504

† Zero: 100

a Wet digestion, although not a paragraph 8 exclusion, was

not studied because of the lack of plant-specific data.

Emulsion and solution crumb rubber, although candidates

for exclusion, were studied, because data were available.

Source: USEPA.

Table 2 BPT Limitations for Subcategories of Rubber Processing Industry (kg/kkg of raw material)

Tire and innertube plantsb

Emulsion crumbrubber

Solution crumb

Pollutant

Dailymax

30-dayavg.a

Dailymax

30-dayavg.a

Dailymax

30-dayavg.a

Dailymax

30-dayavg.a

Dailymax

30-dayavg.a

Dailymax

30-dayavg.a

Pan, dry digestion,mechanical

Dailymax

30-dayavg.a

Dailymax

30-dayavg.a

Dailymax

30-dayavg.a

Dailymax

30-dayavg.a

Dailymax

30-dayavg.a

a Computed from average daily value taken over 30 consecutive days.

b Oil and grease limitations for nonprocess wastewater from plants placed in operation before 1959: daily max ¼ 10 mg/L; 30-day avg ¼ 5 mg/L.

c General molded, extruded, and fabricated rubber.

d Limitation is 6 – 9 pH units for all subcategories.

e Latex-dipped, latex-extruded, and latex-molded goods.

After compounding, the stock is sheeted out in a roller mill and extruded into sheets orpelletized This new rubber stock is tacky and must be coated with an antitack solution, usually asoapstone solution or clay slurry, to prevent the sheets or pellets from sticking together duringstorage.

The rubber stock, once compounded and mixed, must be molded or transformed into theform of one of the final parts of the tire This consists of several parallel processes by which thesheeted rubber and other raw materials, such as cord and fabric, are made into the followingbasic tire components: tire beads, tire treads, tire cords, and the tire belts (fabric) Tire beads arecoated wires inserted in the pneumatic tire at the point where the tire meets the wheel rim (onwhich it is mounted); they ensure a seal between the rim and the tire The tire treads are the part

of the tire that meets the road surface; their design and composition depend on the use of the tire.Tire cords are woven synthetic fabrics (rayon, nylon, polyester) impregnated with rubber; theyare the body of the tire and supply it with most of its strength Tire belts stabilize the tires andprevent the lateral scrubbing or wiping action that causes tread wear

The processes used to produce the individual tire components usually involve similarsteps First, the raw stock is heated and subjected to a final mixing stage before going to a rollermill The material is then peeled off rollers and continuously extruded into the final componentshape Tire beads are directly extruded onto the reinforcing wire used for the seal, and tire belt isproduced by calendering rubber sheet onto the belt fabric

The various components of the tire are fitted together in a mold to build green, or uncured,tires which are then cured in an automatic press Curing times range from less than one hour forpassenger car tires to 24 hours for large, off-the-road tires After curing, the excess rubber on thetire is ground off (deflashed) to produce the final product

This subcategory is often subdivided into two groups of plants: (a) those startingoperations prior to 1959 (applies to 39 plants) and (b) those starting operations after 1959 Thissubdivision must be recognized in applying limitations on plant effluents of oil and greasebecause BPT limitations are different for the two groups of plants For plants placed in operationafter 1959, the 30-day average oil and grease limitation is 0.016 kg/kkg of product For plantsplaced in operation prior to 1959, the limitation is the same (0.016 kg/kkg) but only for processwastewater Process wastewater for these pre-1959 plants comes from soapstone solutionapplications, steam cleaning operations, air pollution control equipment, unroofed process oilunloading areas, mold cleaning operations, latex applications, and air compressor receivers.Water used only for tread cooling and discharges from other areas of such plants is classified asnonprocess wastewater, in which oil and grease levels are limited to 5 mg/L as a 30-day averageand 10 mg/L as a daily maximum

Emulsion polymerization, the traditional process for synthetic rubber production, is thebulk polymerization of droplets of monomers suspended in water Emulsion polymerization isoperated with sufficient emulsifier to maintain a stable emulsion and is usually initiated byagents that produce free radicals This process is used because of the high conversion and thehigh molecular weights that are possible Other advantages include a high rate of heat transferthrough the aqueous phase, easy removal of unreacted monomers, and high fluidity at highconcentrations of product polymer Over 90% of styrene butadiene rubber (SBR) is produced bythis method Approximately 17 plants use the emulsion crumb rubber process

Raw materials for this process include styrene, butadiene, catalyst, activator, modifier, andsoap solution

Polymerization proceeds stepwise through a train of reactors This reactor systemcontributes significantly to the high degree of flexibility of the overall plant in producingdifferent grades of rubber The reactor train is capable of producing either “cold” (277 – 280 K,

103 – 206 kPa) or “hot” (323 K, 380 – 517 kPa) rubber The cold SBR polymers, produced at the

lower temperature and stopped at 60% conversion, have improved properties when compared tohot SBRs The hot process is the older of the two For cold polymerization, the monomer –additive emulsion is cooled prior to entering the reactors Each reactor has its own set of coolingcoils and is agitated by a mixer The residence time in each reactor is approximately one hour.Any reactor in the train can be bypassed The overall polymerization reaction is ordinarilycarried to no greater than 60% conversion of monomer to rubber since the rate of reaction fallsoff beyond this point and product quality begins to deteriorate The product rubber is formed inthe milky white emulsion phase of the reaction mixture called latex Short stop solution is added

to the latex exiting the reactors to quench the polymerization at the desired conversion Thequench latex is held in blowdown tanks prior to the stripping operation

The stripping operation removes the excess butadiene by vacuum stripping, and thenremoves the excess styrene and water in a perforated plate stripping column The water andstyrene from the styrene stripper are separated by decanting and the water is discharged to thetreatment facility The recovered monomers are recycled to the monomer feed stage The latex isnow stabilized and is precipitated by an electrolyte and a dilute acid This coagulation impartsdifferent physical characteristics to the rubber depending on the type of coagulants used Carbonblack and oil can be added during this coagulation/precipitation step to improve the properties

of the rubber This coagulated crumb is separated from the liquor, resuspended and washed withwater, then dewatered, dried, and pressed into bales for shipment The underflow from thewashing is sent to the wastewater treatment facility

Subcategory 3: Solution Crumb Rubber Production

Solution polymerization is bulk polymerization in which excess monomer serves as the solvent.Solution polymerization, used at approximately 13 plants, is a newer, less conventional processthan emulsion polymerization for the commercial production of crumb rubber Polymerizationgenerally proceeds by ionic mechanisms This system permits the use of stereospecific catalysts

of the Ziegler – Natta or alkyl lithium types which make it possible to polymerize monomers into

a cis structure characteristic that is very similar to that of natural rubber This cis structure yields

a rubbery product, as opposed to a trans structure which produces a rigid product similar toplastics

The production of synthetic rubbers by solution polymerization processes is a stepwiseoperation very similar in many aspects to production by emulsion polymerization Thereare distinct differences in the two technologies, however For solution polymerization, themonomers must be extremely pure and the solvent should be completely anhydrous In contrast

to emulsion polymerization, where the monomer conversion is taken to approximately 60%,solution polymerization systems are polymerized to conversion levels typically in excess of90% The polymerization reaction is also more rapid, usually being completed in 1 to 2 hours.Fresh monomers often have inhibitors added to them while in storage to prevent prematurepolymerization These inhibitors and any water that is present in the raw materials must beremoved by caustic scrubbers and fractionating drying columns to provide the solution processwith the high purity and anhydrous materials needed The purified solvent and monomers arethen blended into what is termed the “mixed feed,” which may be further dried in a desiccantcolumn

The dried mixed feed is now ready for the polymerization step, and catalysts can be added

to the solution (solvent plus monomers) just prior to the polymerization stage or in the leadpolymerization reactor

The blend of solution and catalysts is polymerized in a series of reactors The reaction ishighly exothermic and heat is removed continuously by either an ammonia refrigerant or by

chilled brine or glycol solutions The reactors are similar in both design and operation to thoseused in emulsion polymerization The mixture leaves the reactor train as a rubber cement, that is,polymeric rubber solids dissolved in solvent A short stop solution is added to the cement afterthe desired conversion is reached.

The rubber cement is then sent to storage tanks where antioxidants and extenders aremixed in The rubber cement is pumped from the storage tank to the coagulator where the rubber

is precipitated with hot water under violent agitation The solvent and unreacted monomer arefirst steam stripped overhead and then condensed, decanted, and recycled to the feed stage.The bottom water layer is discharged to the wastewater treatment facility

The stripped crumb slurry is further washed with water, then dewatered, dried, and baled

as final product Part of the water from this final washing is recycled to the coagulation stage, andthe remainder is discharged for treatment

Subcategory 4: Latex Rubber Production

The emulsion polymerization process is used by 17 production facilities to produce latex rubberproducts as well as solid crumb rubber Latex production follows the same processing steps asemulsion crumb rubber production up to the finishing process Between 5 and 10% of emulsionpolymerized SBR and nearly 30% of nitrile rubber production (NBR) are sold as latex Latexrubber is used to manufacture dipped goods, paper coatings, paints, carpet backing, and manyother commodities

Monomer conversion efficiencies for latex production range from 60% for temperature polymerization to 98% for high-temperature conversion

low-The monomers are piped from the tank farm to the caustic soda scrubbers where theinhibitors are removed Soap solution, catalysts, and modifiers are added to produce a feedemulsion which is fed to the reactor train Fewer reactors are normally used than the numberrequired for a crumb product line When polymerization is complete, the latex is sent to aholding tank where stabilizers are added

A vacuum stripper removes any unwanted butadiene, and the steam stripper following itremoves the excess styrene Neither the styrene nor butadiene is recycled Solids are removedfrom the latex by filters, and the latex may be concentrated to a higher solids level

Subcategories 5, 6, 7: Small-, Medium-, and Large-Sized General Molding,

Extruding, and Fabricating Plants

These three closely related subcategories are divided based on the volume of wastewateremanating from each These subcategories include a variety of processes such as compressionmolding, transfer molding, injection molding, extrusion, and calendering An estimated 1385plants participate in these subcategories

A common step for all of the above processes is the compounding and mixing of theelastomers and compounding ingredients The mixing operation is required to obtain a thoroughand uniform dispersion of the rubber and other ingredients Wastewater sources from the mixingoperation generally derive from leakage of oil and grease from the mixers

Compression molding is one of the oldest and most commonly used manufacturingprocesses in the rubber fabrication industry General steps for the processes include warming theraw materials, preforming the warm stock into the approximate shape, cooling and treating withantitack solution, molding by heat and pressure, and finally deflashing Major products from thisprocess include automotive parts, medical supplies, and rubber heels and soles

Transfer molding involves the forced shifting of the uncured rubber stock from one part

of the mold to another The prepared rubber stock is placed in a transfer cavity where a ram

forces the material into a heated mold The applied force combined with the heat from the moldsoftens the rubber and allows it to flow freely into the entire mold The molded item is cured,then removed and deflashed Final products include V-belts, tool handles, and bushings withmetal inserts.

Injection molding is a sophisticated, continuous, and essentially automatic process thatuses molds mounted on a revolving turret The turret moves the molds through a cyclic processthat includes rubber injection, curing, release agent treatment, and removal Deflashing occursafter the product has been removed A wide range of products is made by this process, includingautomotive parts, diaphragms, hot-water bottles, and wheelbarrow tires

The extrusion process takes unvulcanized rubber and forces it trough a die, which results

in long lengths of rubber of a definite cross-section There are two general subdivisions of thistechnique; one extrudes simple products and the other builds products by extruding the rubberonto metal or fabric reinforcement Products from these techniques include tire tread, cablecoating, and rubber hose

Calendering involves passing unformed or extruded rubber through a set or sets of rolls toform sheets or rolls of rubber product The thickness of the material is controlled by the spacebetween the rolls The calender may also produce patterns, double the product thickness bycombining sheets, or add a sheet of rubber to a textile material The temperature of the calenderrolls is controlled by water and steam Products produced by this process include hospitalsheeting and sheet stock for other product fabrication

This subcategory represents a process that is used to recover rubber from fiber-bearingscrap Scrap rubber, water, reclaiming and defibering agents, and plasticizers are placed in asteam-jacketed, agitator-equipped autoclave Reclaiming agents used to speed up depolymer-ization include petroleum and coal tar-base oils and resins as well as various chemical softenerssuch as phenol alkyl sulfides and disulfides, thiols, and amino acids Defibering agentschemically do the work of the hammer mill by hydrolyzing the fiber; they include caustic soda,zinc chloride, and calcium chloride

A scrap rubber batch is cooked for up to 24 hours and then discharged into a blowdowntank where water is added to facilitate subsequent washing operations Digester liquor isremoved by a series of screen washings The washed rubber is dewatered by a press and thendried in an oven Two major sources of wastewater are the digester liquor and the washwaterfrom the screen washings

Two rubber reclaiming plants use the wet digestion method for reclamation of rubber

Subcategory 9: Pan, Dry Digestion, and Mechanical Reclaimed Rubber

This subcategory combines processes that involve scrap size reduction before continuing thereclaiming process The pan digestion process involves scrap rubber size reduction on steel rolls,followed by the addition of reclaiming oils in an open mixer The mixture is discharged intoopen pans, which are stacked on cars and rolled into a single-cell pressure vessel where livesteam is used to heat the mixture Depolymerization occurs in 2 to 18 hours The pans are thendischarged and the cakes of rubber are sent on for further processing The steam condensate ishighly contaminated and is not recycled

The mechanical rubber reclaiming process, unlike pan digestion, is continuous andinvolves fiber-free scrap being fed into a horizontal cylinder containing a screw that works thescrap against the heated chamber wall Reclaiming agents and catalysts are used fordepolymerization As the depolymerized rubber is extruded through an adjustable orifice, it isquenched The quench vaporizes and is captured by air pollution control equipment Thecaptured liquid cannot be reused and is discharged for treatment

Subcategory 10: Latex-Dipped, Latex-Extruded, and Latex-Molded Goods

These three processes involve the use of latex in its liquid form to manufacture products Latexdipping consists of immersing an impervious male mold or article into the latex compound,withdrawing it, cleaning it, and allowing the adhering film to air dry The straight dip process isreplaced by a coagulant dip process when heavier films are desired Fabric or other items may bedipped in latex to produce gloves and other articles When it has the required coating, the mold isleached in pure water to improve physical and electrical properties After air drying, the itemsare talc-dusted or treated with chlorine to reduce tackiness Water is often used in severalprocesses, for makeup, cooling, and stripping Products from dipping include gloves, footwear,transparent goods, and unsupported mechanical goods

Latex molding employs casts made of unglazed porcelain or plaster of paris The molds aredusted with talc to prevent sticking The latex compound is then poured into the mold andallowed to develop the required thickness The mold is emptied of excess rubber and then ovendried The mold is removed and the product is again dried in an oven Casting is used tomanufacture dolls, prosthetics, printing matrices, and relief maps

Subcategory 11: Latex Foam

No latex foam facilities are known to be in operation at this time

12.1.2 Wastewater Characterization

The raw wastewater emanating from rubber manufacturing plants contains toxic pollutants thatare present due to impurities in the monomers, solvents, or the actual raw materials, or areassociated with wastewater treatment steps Both inorganic and organic pollutants are found inthe raw wastewater, and classical pollutants may be present in significant concentrations.Wastewater from reclaimed rubber manufacturing had 16,800 – 63,400 mg/L total solids,

1000 – 24,000 mg/L suspended solids, 3500 – 12,500 mg/L BOD (biochemical oxygendemand), 130 – 2000 mg/L chlorides, pH of 10.9 – 12,2, while wastewaters from syntheticrubber manufacturing had 1900 – 9600 mg/L total solids, 60 – 3700 mg/L suspended solids,

75 – 1600 mg/L BOD, and pH of 3.2 – 7.9 [3]

facilities in each subcategory (no data are available for Subcategories 9, 10, and 11).Table 4

gives a subcategory profile of the pollutant loadings (no data are available for Subcategories 8,

10, and 11) These tables were prepared from available screening and verification sampling data.The minimum detection limit for toxic pollutants is 10 mg/L and any value below 10 mg/L ispresented in the following tables as BDL, below detection limit

In-plant management practices may often control the volume and quality of the treatmentsystem influent Volume reduction can be attained by process wastewater segregation fromnoncontact water, by recycling or reuse of noncontact water, and by the modification of plantprocesses Control of spills, leakage, washdown, and storm runoff can also reduce the treatmentsystem load Modifications may include the use of vacuum pumps instead of steam ejectors,recycling caustic soda solution rather than discharging it to the treatment system, andincorporation of a more efficient solvent recovery system

12.1.3 Tire and Inner Tube Manufacturing

The tire and inner tube manufacturing industry has several potential areas for wastewaterproduction, but water recycle is used extensively The major area for water use is in processes

Table 3 Concentrations of Toxic Pollutants Found in the Rubber Processing Industry by Subcategory, Verification, and Screening

Toxic pollutants (mg/L)

Tire and inner tube manufacturing

Number of

Number of

Metals and Inorganics

Table 3 Continued

Toxic pollutants (mg/L)

Tire and inner tube manufacturing

Table 3 Continued

Toxic pollutants (mg/L)

Tire and inner tube manufacturing

Analytic methods: V.7.3.29, Data sets 1,2.

BDL, below detection limit.

a

40 mg/L of trichloroethylene also measured in city water.

b

Detection limit of acrylonitrile by direct aqueous injection was 2300 mg/L.

c This value believed to be a glassware contaminant.

d These pollutants appear to be attributed to tire operation.

e Wastewater is from both tire and reclaiming processes.

Table 4 Industry Profile of Toxic and Classical Pollutant Loadings, Verification, and Screening Data (Toxic Pollutants Kg/kkg)

Tire and inner tube manufacturing

Table 4 Continued

Tire and inner tube manufacturing

Latex rubber manufacturingMetals and Inorganics

Metals and Inorganics

Table 4 Continued

Tire and inner tube manufacturing

Analytic methods: V.7.3.29, Data sets 1,2.

BDL, below detection limit.