Valorisation of spent foundry sand in construction applications

Spent foundry sands are classified as non-hazardous, noninert waste due to their leaching behaviour and, being a waste, they have to be handled by an authorized waste manager, all of which restricts their valorisation possibilities. LIFE ECO-SANDFILL project would overcome those barriers by introducing a sand reclamation system in the value chain in order to obtain a specified quality sand by-product with low potential for adverse environmental impact.

Spent Foundry Sand (SFS) generation is intrinsic to metal casting processes. Most ferrous foundries use sand moulds and cores for casting, which are discarded after pouring (lost mould technique). This generates large amounts of waste sand, representing approximately 60-85% of total solid waste from foundry.

Foundries use two differentiated types of sand moulding: green sand and chemically-bonded sand moulding. Green sand mixture is made up of silica sand, water and bentonite clay, acting as binder, with silica content of around 85-95%. Green sand moulding is the most common moulding technique, representing about 75% of total foundry sand.

FCC Aqualia central, Inodoro

In the chemically-bonded sand, silica sand is mixed with a resin and a catalyst initiates the reaction for setting/ hardening the mixture. Chemical sands have silica contents of over 95%. Therefore, Spent Foundry Sand consists primarily of silica sand (>85%), coated with a thin film of burnt carbonaceous and residual binder (bentonite, resins, etc.). The impurities usually comprise aluminium, calcium, sodium, carbon and potassium components, in total below 12%.

According to statistics from CAEF (The European Foundry Association), 4616 ferrous and non-ferrous foundry units were active in 2015 in Europe, with total yearly production of 15.4 Mt of metal castings.

There are no accurate records on a European level of annual
amounts of ferrous and nonferrous foundry sand waste, but based on public statistics on non-hazardous waste by industrial activity and studies carried out by industry sector associations, the volume of SFS generated by ferrous foundries in Europe can be estimated at around 4.5-6 Mt/yr. Only 25-30% of that waste sand is recovered in a few applications, mainly in the cement industry, agricultural soils and landfill covering.

However, those do not constitute stable solutions viable for all the waste sand generated, and the remaining percentage is disposed of in landfills.

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Published in: FuturENVIRO #44 October2017