Towards a more sustainable foundry

Over the past four decades, foundries have significantly stepped up their recycling efforts. However, materials such as filter dust, sand, slag and refractory materials are still largely disposed of in landfill sites, often at relatively low cost. In recent years, environmental regulations and a reduction in landfill capacity have led to higher disposal costs and greater liability risks. To make matters worse, some foundry waste is now classified as hazardous, which further increases the financial and regulatory pressure on the industry.

Foundry waste

The UK government has set ambitious targets to reduce municipal landfill waste to less than ten per cent by 2035 and to near zero by 2045. These targets are intended to force local authorities to close landfills or face penalties for non-compliance. Consequently, the disposal of foundry waste via general landfills is becoming increasingly difficult and expensive.

Currently, foundry waste is subject to a reduced landfill tax of £4.05 (approx. €4.60) per tonne, compared with the standard rate of £126.15 (approx. €143.26), provided its loss on ignition (LOI) is below 10 per cent. However, the government is currently conducting consultations and may abolish this reduced tax rate. Should this be implemented, disposal costs could rise thirty-fold, which would significantly increase the financial pressure on the foundry industry.

Waste Acceptance Criteria (WAC) and classification issues

Whilst the exemption from the landfill tax for foundry sand offers some relief, it has no bearing on the WAC, which determine whether waste is classified as inert, non-hazardous or hazardous. Foundry sand is increasingly being classified as hazardous as it contains high levels of dissolved organic carbon (DOC) – over 800 mg/kg – or phenols. The disposal costs for such hazardous waste can range from 45.00 to 600.00 pounds (between 51.12 and 681.60 euros) per tonne, depending on the severity of the contamination.

Hazardous sand waste cannot be reused without further treatment. Many chemically-bonded sand foundries that use simple dry-sanding recovery methods produce sand that exceeds these limits. Although these regulations have been in force for some time, their enforcement has varied considerably from site to site. This inconsistency is now changing, and foundries are facing a sharp rise in disposal costs – a trend that is likely to continue unless proactive measures are taken. Foundries must review their processes to reduce the volume of waste generated.

Green sand recovery

Green sand systems are complex and require a precise balance between multiple inputs and outputs (see process map). As castings become increasingly complex and core-intensive, the amount of fresh sand and core sand has risen, leading to more waste and a greater demand for bentonite and carbon-containing additives. This change has driven up both material and disposal costs.

To counteract this, recovery systems have been developed to recover waste sand, bentonite and carbon-containing materials. In recent years, several technologies have been developed to clean waste sand and return it to the core workshop, thereby significantly reducing the consumption of new sand and the burden on landfill sites.

However, non-thermal recovery processes can generate significant quantities of dust with a LOI of over ten per cent, making them unsuitable for landfill under current regulations. Thanks to recent innovations in rehydrating the high proportion of bentonite and carbon-containing materials in the dust, these can now be returned to the green sand system, thereby reducing waste and raw material costs. Alternatively, the material must be mixed with waste green sand in a dilution process to keep the total LOI below ten per cent; however, this leads to a reduction in the total quantities recovered. The type of recovery system – whether cold or involving a thermal component – should be carefully assessed based on the nature of the waste generated.

Collaborative solutions and cost-effective strategies for waste reduction

Given the high capital costs associated with recovery technologies, small and medium-sized green sand foundries can benefit from partnerships or clusters with similar operations. By sharing costs and making joint investments, these groups can explore more cost-effective methods for recycling waste materials – for example, incorporating green sand into asphalt and other construction products. In fact, green sand waste is already being used as a capping material at some landfill sites. To ensure the success of any strategy for beneficial reuse, it is essential to maintain consistency and prevent cross-contamination within the waste stream. The era of indiscriminate waste disposal is rapidly drawing to a close.

Furthermore, there are several cost-effective, practical measures that can be taken to reduce the overall volume of waste:

• Refine core designs by reducing bulky, oversized cores, e.g. by hollowing out heavy cores.
• Monitor and control the LOI and methylene blue clay content to avoid unnecessary overdosing.
• Recycle waste core sand and core residues from screening using simple dry-scraping recovery methods.
• Improve planning to reduce the number of tool changes, thereby minimising the generation of mixed core sand waste and core scrap.
• Regularly test the fine fractions from the dust separator for methylene blue content and LOI to ensure that the sand cooling and extraction systems are functioning optimally.
• Regularly inspect cyclones and collection hoppers to ensure that no excessive fines are being drawn into the bag filters.

Core package casting and the move away from green sand

In recent years, some foundries – particularly those producing castings such as lorry engine blocks and cylinder heads – have completely abandoned green sand processes. This development has led to facilities that produce only minimal, if any, sand waste. Most of these foundries use large core shooting machines to blow phenolic urethane cores, creating integrated core packages which are then inserted into a support system for casting.

By utilising thermal recovery technologies, these foundries achieve a high degree of sand reuse whilst drastically reducing waste. Furthermore, this level of efficiency opens up the possibility of replacing conventional quartz sand with ceramic sands such as Cerabeads, thereby eliminating serious health and safety risks associated with respirable crystalline quartz sand.

This transition also enables better control of casting tolerances and greater flexibility in casting design, particularly in light of recent advances in rapid 3D sand printing and cellular manufacturing. Overall, the core package approach represents a significant step towards a more sustainable and adaptable foundry model.

Challenges for no-bake foundries

Most contract foundries in the United Kingdom rely on mechanical wear in conjunction with organic binders, predominantly alkaline phenol-furan or phenol-urethane systems. Recovery rates for these systems typically range between 60 and 90 per cent, with a large proportion of the waste sand classified as hazardous due to the presence of leachable organic compounds such as phenols.

The dust generated during the classification process is also hazardous, which complicates disposal and presents challenges in terms of compliance with environmental regulations. Furthermore, the high concentration of organic compounds in the waste makes it difficult to reuse or recycle through viable reuse pathways, which presents a growing problem for foundries that rely on these organic binder systems. Recovery rates can be increased to over 90 per cent by introducing secondary comminution, incorporating thermal recovery as part of the process, or completely replacing mechanical comminution.

Considerations regarding thermal recovery and the binder process

Thermal recovery, in conjunction with the phenol-urethane process, offers the most effective method for disposing of hazardous sand waste, as it completely removes organic compounds from the sand. The result is a clean, reusable material that requires virtually no classification as hazardous waste.

However, each binder system presents its own challenges:

• Furan binders: Whilst the thermal regeneration process for furan systems is effective, it produces sulphur emissions which may raise environmental concerns and require mitigation measures.
• Alkaline phenolic binders: Regeneration rates exceeding 90 per cent can lead to an accumulation of potassium and sodium oxides in the sand. This accumulation can impair dimensional stability and reduce the sand’s sintering temperature, which may affect casting quality. To ensure optimum performance, the addition of up to ten per cent fresh sand is usually required.

Despite these limitations, sand processed by thermal recovery – regardless of the type of binder – is no longer classified as hazardous, which significantly simplifies disposal and improves the scope for meaningful reuse.

No-bake foundries and innovations in inorganic binders

Traditionally, many British contract foundries have relied on organic binder systems in combination with simple mechanical crushing for sand preparation, which produces hazardous waste sand whose disposal is becoming increasingly complex and costly, and whose potential for meaningful reuse is limited.

Consequently, such inorganic binder systems are gaining prominence as a more sustainable alternative. Although they often achieve lower recovery rates when using standard dry abrasion, the resulting sand is non-hazardous and significantly easier to reuse or repurpose.

This offers foundries the opportunity to avoid these burdens without significant capital investment. Furthermore, a dual mechanical sand-sifting process combined with drying can achieve recovery rates of eighty to ninety per cent when using the inorganic binders from John Winter & Co Ltd.

Other foundry waste and recovery potential

In addition to sand waste, foundries generate by-products from melting, blasting and cleaning processes. These waste streams may contain untapped value and require more intelligent management:

• Fine dust collected by dust separators during electrosmelting can often be reused. For example, fumes from galvanised steel scrap contain zinc oxide, which can be extracted and sold for industrial purposes.
• Dusts rich in silica can have significant potential for reuse in the glass and ceramics industries.
• To preserve this potential for reuse, it is crucial to separate heavy metals from the dust to prevent contamination of other waste streams.
• Smaller foundries can benefit from pooling resources to create viable pathways for the reuse of filter dust from smelting operations.
• Aluminium dross can be used for metal recovery; however, economies of scale in turn argue in favour of collaborative efforts.
• The use of John Winter fluxes helps to reduce aluminium carry-over into the slag and minimise melting losses.
• Some slag by-products are suitable for construction applications, but separation is crucial – furnace waste containing heavy metals must be kept separate.
• Working with customers to phase out heavy-metal alloys (e.g. lead-containing red brass alloys). In the USA, bismuth is now the preferred choice in copper alloys, thereby virtually eliminating lead.

Material selection to promote reuse

To promote the reuse of waste and reduce environmental hazards, foundries should consider the following approaches:

• Non-crystalline silicon dioxide refractory materials, which facilitate safe disposal and reuse.
• Ceramic sands that reduce the crystalline silica content in blasting and finishing dusts.
• Good operating practices to minimise chemical waste and waste oil, ideally in collaboration with suppliers via recycling or take-back schemes.
• Switching to non-hazardous materials throughout the process wherever possible.
• Regular audits of all waste streams to identify opportunities for reuse or safe treatment options.

Air emissions and sustainable alternatives

Foundries in Europe are under increasing pressure to control volatile organic compounds (VOCs) and airborne pollutants such as formaldehyde. Some sites have installed post-combustion systems, but these have a high carbon footprint.

More sustainable alternatives are emerging

• Inorganic binders are now widely used in the aluminium sector and are being tested in iron casting applications to reduce emissions.
• Products such as JW’s Winterbond and Geopol inorganic cold-box systems contain no VOCs and do not produce hazardous moulding sands – making them ideal for reuse.
• JW’s zero-VOC release agents for green sand, including:

• PF880 (hydrocarbon-based)
• PF990 (plant-based), which significantly reduces the carbon footprint.

By implementing these materials, foundries can almost completely eliminate air emissions and odours.

Conclusion: Managing foundry sand waste

• Increase the use of opportunities for beneficial reuse.
• Invest in advanced recovery technologies.
• Adopt non-hazardous processes to enable safe reuse.
• Continuously review and optimise sand waste management.

Conclusion: Managing metallurgical and process waste

• Separate waste containing heavy metals to prevent cross-contamination.
• Work with customers to eliminate hazardous alloying elements.
• Use refractory materials with a low silica content to reduce air pollution.
• Recover and reuse aluminium dross and slag.
• Minimise smelting losses by using suitable fluxes.
• Minimise chemical waste – work with suppliers on take-back schemes.

Key strategic message

Above all, foundries should prioritise non-hazardous production, optimisation of waste streams and collaborative approaches – such as cluster formation – to make sustainable practices economically viable.