Chemical manufacturer benefits by reducing waste A Case Study at Dow Corning Ltd

This Case Study demonstrates the cost and environmental benefits of reducing

hazardous waste through improved resource management, chemical substitution and

the effective re-use of by-products. Silicone specialist, Dow Corning Ltd, used the waste

hierarchy as the basis of its approach to minimising hazardous waste at its Barry site in

South Wales. A series of initiatives involving a number of processes looked first at

eliminating and reducing waste at source, followed by innovative by-product recovery

and re-use. The benefits of the various hazardous waste minimisation initiatives at

Dow Corning’s Barry site include:

Significant savings from reduced raw material and waste disposal costs

By-product recovery for re-use in the manufacturing process

Elimination and substitution of hazardous solvents

Less waste to sewer and landfill

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Background

Site ProcessesThe Dow Corning site at Barry in South Walesmanufactures silicones and silicone intermediates.Silicones can be manufactured as either fluids orelastomeric products for uses including lubrication,sealing, bonding, releasing and defoaming.

Various feedstock ingredients are produced at the site byreacting silicon with methyl chloride in fluidised bedreactors to give a mixture of methyl chlorosilanes (seeFig 1). The main product, dimethyldichlorosilane, isseparated out and hydrolysed to produce a crudesilicone polymer which is used in downstream finishingprocesses that lead to the generation of over 7,000products. Hydrogen chloride (HCl) is a by-product of thehydrolysis reaction. This is recovered (see below) andreacted with methanol to produce more methylchloride.

Hazardous Waste MinimisationInitiatives Dow Corning examined its main manufacturing processand various finishing processes for opportunities toreduce hazardous waste through:

n better use of resources;

n raw material substitution;

n greater by-product re-use and recycling.

By-product Exchange

One of the main success stories over the last few yearshas been the symbiotic relationship of Dow Corningwith a neighbouring company on the same industrialpark, Cabot Carbon Limited. Monomethylchlorosilane,one of the by-products from the fluidised bed reactorin the main manufacturing process, is exported byinterconnecting pipework to Cabot Carbon where it isused in the manufacture of fumed amorphous silica.Some of this product is fed back to Dow Corning tomanufacture elastomer products.

Gases from the fumed amorphous silica process arecleaned by Cabot Carbon via a recovery system,including a series of scrubbers. The scrubber liquors,consisting mainly of HCl, are returned to Dow Corningfor re-use in the production of methyl chloride. Thisreduces the amount of chloride-based raw materialthat Dow Corning would otherwise have to buy.

During periods of no methyl chloride production,Cabot Carbon was historically unable to feed back HCl.Installation of an intermediate buffer tank provided anuninterrupted supply of HCl and enabled Dow Corningto reduce the number of road tanker deliveries ofmethyl chloride raw material by 23%.

HCl Recovery from a Finishing Process

An intermediate product, trimethylchlorosilane, is usedin a finishing process to produce hexamethyldisiloxane.The chlorides generated from this process used to beneutralised with lime, with the liquors from thereaction being discharged to sewer after filtration andthe filter cake landfilled. Now, HCl is recovered and re-used in the methyl chloride production reaction,thus eliminating the need for the neutralisation andfiltration processes. The revised process has reducedtanker deliveries of methyl chloride raw material by afurther 11%.

Elimination of Hazardous Spent Filter Cartridges

Historically, all spent filter cartridges from the finishingprocesses were placed in one hazardous waste skip foroff-site disposal. However, it was discovered that onlyone process actually produced spent cartridges thatwere hazardous waste. This was because a filter aidused to remove spent catalyst was classed as hazardous.

The process was changed to incorporate a liquidcatalyst, which is inactivated at the end of the reaction(becoming inert) and is left in the product. This changeeliminated the need for the filtration process and thususe of the filter aid. As a result, Dow Corning savedover £10,000/year in reduced waste disposal costs asthe skip for spent filter cartridges could now be classedas non-hazardous. In addition, the plant’s capacity hasincreased by 15% with the removal of the filtrationstep between batches.

Extending Lifetime of Molecular Sieve

Another site process uses a molecular sieve to removeimpurities from the crude silicone polymer. Asrecommended by its supplier, Dow Corning used topurge the molecular sieve material with cold nitrogengas. In 2002, this resulted in 80 tonnes of waste material.The site carried out a successful trial using an alternativepurging technique. This change prolonged the life of themolecular sieve and the site now disposes of only 8tonnes/year, saving over £10,000/year.

Dow Corning Corporation is a global leader in silicon-based technology and innovation. Its policy of sustainabilitythroughout its operations is driven by legislative requirements, spiralling disposal costs, and a commitment tocorporate and social responsibility. The approach taken was to move up the waste hierarchy, with particularemphasis on eliminating and reducing waste at source. The company assessed the wastes produced andimplemented initiatives to minimise both hazardous waste arisings and costs.

Silicon metal

Silo

xan

es

Methanol

Silicon powder

HCl Ch

loro

sila

nes

Grinder

Methylchloride

Waterwash

Reactor

Distillation

Polymerisation Products

Fig 1 Schematic of the manufacturing process

Elimination of Cyclohexane

Cyclohexane was previously used as a solvent in themanufacture of a silicone-based catalyst used in anelastomer finishing process. This catalyst has beensubstituted with another silicone-based catalystproduced on-site without the need for cyclohexane.Eliminating the use of cyclohexane, a hazardoussubstance, has saved Dow Corning several thousandpounds annually in reduced raw material and disposalcosts.

Substitution of Toluene

Dow Corning has minimised the use of toluene as aclean-out solvent. One of the site's raw materials issupplied contaminated with xylene which wasextracted and collected in drums as a waste material.The extracted xylene is now re-used in place oftoluene, saving over £25,000/year and reducing VOCemissions to air by 280 kg/year. Extracted xylene is nolonger routinely disposed of as a waste material.

Cost and Environmental BenefitsThe various initiatives implemented by Dow Corning toeliminate and minimise hazardous waste at its Barrysite have resulted in significant cost and environmentalbenefits (see Table 1). Dow Corning continues toinvestigate ways of achieving more savings throughprocess changes and eliminating the use of hazardousmaterials.

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Table 1 Summary of cost and environmental benefits

Initiative Cost benefit Environmental benefit

By-product exchange n 23% reduction in raw n No need to tanker by-product off-site(recovery of HCl) n methyl chloride costs n Scrubber liquors re-used

Recovery of HCl from a n 11% reduction in raw n No discharge to sewerfinishing process n methyl chloride costs n Solid waste to landfill eliminated

Spent filter cartridges n >£10,000/year n Eliminated source of hazardous wasten 15% increase in capacity

Increased life of n Ten-fold reduction in purchase n Reduced raw material consumptionmolecular sieve n and waste disposal costs n Less hazardous waste produced

Elimination of cyclohexane n Several thousand pounds annually n Use of hazardous substance eliminated

Substitution of toluene n >£25,000/year n Minimised toluene usen Reduced VOC emissions by 280 kg/year n Xylene no longer routinely disposed of

as waste

For further informationplease contact the

Environmentand EnergyHelpline0800 585794© Crown copyright. First printed June 2007. Printed on paper containing 80% recycled post-consumer fibre.

This material may be freely reproduced in its original form except for sale or advertising purposes.

Harwell Science and Innovation Campus | Didcot | Oxfordshire | OX11 0QJE-mail: helpline@envirowise.gov.uk Internet: www.envirowise.gov.uk

Envirowise - Practical Environmental Advice for Business - is a Government programme that offers free, independent and practical adviceto UK businesses to reduce waste at source and increase profits. It is managed by Momenta, an operating division of AEA Technology plc,and Serco TTI. Envirowise is funded across the UK by the DTI and Defra and the Business Resource Efficiency and Waste (BREW)Programme in England, the Scottish Executive in Scotland and the Materials Action Programme (MAP) in Wales. This publication wasprepared with assistance from ARL Consulting Ltd.

Useful publicationsfrom Envirowise(GG490) - Practical ways tomanage and minimisehazardous waste

(GG681) - Reducing hazardouswaste in the chemical andpharmaceutical industries

(GG395) - Better solventmanagement in specialitychemicals manufacture

Dow Corning LtdDow Corning is a global manufacturer of silicone-based materials.Its 65-hectare site at Barry in South Wales employs around 600people in manufacturing, research and development. Barry is anintegrated site developing, commercialising and producingsilicone-based products and intermediates for use by customersworldwide. The company participates in the chemical industry’sResponsible Care® programme and has won a number ofenvironmental awards.

Comments from Dow Corning LtdEliminating and minimising hazardous waste from our production processes, or movingthem up the waste hierarchy to provide better environmental outcomes, fits directlywith our company’s corporate core value of sustainability.

“ Eliminating and minimising hazardous waste fits directly with

our core value of sustainability. ”

Mike Squire, Environmental Team Leader

Host Company:

Dow Corning LtdCardiff RoadBarry Vale of Glamorgan CF63 2YL

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