Foundry sand looks expendable on the shop floor, yet it is one of the largest single items in the cost of a casting. Every cycle takes usable grain out of circulation: some is lost down the dust duct, some is carried out on the shot blast, and some is bonded into the knockout rubble. What comes off the shakeout is partly sintered grain, crushed core sand, metal fines and dust.
Used casting sand returned to the moulding line in that state causes predictable failures. The fine dust consumes binder and water that the grain should have used for strength, permeability falls, mould surfaces suffer from penetration and gas defects, and shakeout takes longer. The sections below set out the equipment and test regime needed to turn spent casting sand into a consistent product.
A reclamation line is a sequence, not a single machine, and the order matters because each stage protects the next from overload. Coarse screening comes first so the crusher and magnetic separator see a size they can handle; dust removal follows, because fine dust reduces permeability most sharply and carries the binder residue; blending comes last.
The first screen is a grizzly or vibrating deck with an aperture sized to pass the sand but reject core scrap and burnt lumps, commonly 4 to 8 mm for green sand and 2 to 4 mm for resin coated sand. A second screen after crushing returns oversize to the mill. The cut size should match the grading the shop needs, since a finer aperture wastes good grain.
Dust is removed in two steps. A cyclone or rotary scrubber takes the bulk of the fine fraction, and a pulse cleaned bag filter polishes the gas stream. Silicone release agents and coating overspray are the worst offenders, being sticky enough to blind filter cloth faster than dry dust. The practical controls are metered release agent application, filter bag inspection each shift, and a recorded differential pressure across the housing.
| Reclamation stage | Typical equipment | Typical duty or cut size | Common failure | Control point |
|---|---|---|---|---|
| Primary screening | Grizzly and vibrating deck | 4 to 8 mm green, 2 to 4 mm resin sand | Blinding by wet fines | Deck inspection each shift |
| Crushing and screen | Impact crusher and screen | Single pass to target grading | Oversize returned to crusher | Screen tension check weekly |
| Dust removal | Cyclone plus bag filter | Filtrate below 10 mg per cubic metre | Filter blinding from overspray | Differential pressure log |
| Metal removal | Overband and drum magnet | Ferrous fines above 0.1 percent | Coated belt loses lift | Fixed cleaning interval |
Metal enters the sand stream from gating and sprue runners, core supports and iron fines broken off castings during knockout. Ferrous contamination is the priority, because iron particles and their oxides interfere with the sintering of reclaimed grains and mark the casting surface. Non-ferrous contamination acts more slowly but accumulates, and it can damage refractory linings in the holding furnace. An overband magnet at the transfer point, with a second at the reclaim feed, removes most metal fines.
The magnet must be matched to the duty. An overband magnet needs a correctly sized belt and a discharge chute that does not choke. A drum magnet must be fed at a controlled layer depth, because a deep bed lets metal hide below the surface. Set belt speed and pole gap so weakly magnetic, finely divided iron is still lifted, since a weak magnet leaves the fines that cause the worst casting defects. Schedule a fixed cleaning interval and keep a ferrous nitrogen measurement on the reclaim.
Hot sand is both a safety and a quality issue. Sand from the shakeout or shot blasting can be well above 80 degrees Celsius, and handling it wet accelerates steam formation, drives moisture into the mixers and shortens the life of resin binders. The reclaim line should include a forced air cooler or water cooled screw before storage, and discharge temperature should be checked at a fixed point each shift.
Storage silos, bins and enclosed conveyors must be sealed against rain. Wetting a reclaim batch changes its flow behaviour and binder demand immediately, and a batch that varies in moisture can never be dosed correctly into the mixer. Silo level sensors, bin activators and a covered transfer point help prevent bridging, a common cause of stoppages and batch variability. Keep separate silos for virgin sand, clean reclaim and high resin sand, since uncontrolled gravity blending produces variable moulds.
The binder is what the reclamation process is ultimately trying to remove from used casting sand. Every cycle leaves a thin film of cured resin, or of a clay and starch carrier, on the grain surface. That film is not inert: it makes grains stick together, the sand harder to sift, and it consumes the new binder at the next mixing. A reclaim whose residual binder level drifts upward will need ever more fresh binder for the same green strength, raising the sand-to-metal ratio.
Two approaches manage this. Mechanical treatment, using an attrition or paddle scrubber, removes the film by abrasion under controlled moisture and is the usual route for resin sand. Thermal treatment, using a fluidised bed or rotary thermal regenerator, burns the binder off; it is more effective but more expensive per tonne and generates combustion gas that must be cleaned. Between cycles the sand is conditioned with a small addition of fresh binder and water. Whichever route is used, record residual binder level against a limit and treat a rising trend as a fault.
A reclamation plant is only as good as its laboratory routine. The sample must represent the blend, so take it from the casting sand reclaim output after cooling and blending, not from a stockpile that has been standing. A composite of grab samples across a shift gives a truer average than a single sample, and the sample must be dried gently so moisture is measured.
| Property | Test method | Indicative reference range | Action when outside range |
|---|---|---|---|
| Moisture content | Oven dry at 105 degrees Celsius | 2 to 4 percent for green sand | Adjust water addition at mixer |
| Permeability | Standard permeability tester | 60 to 120 for grey iron moulds | Check dust carry-over and grading |
| Green compressive strength | Specimen at 300 mm per minute | 130 to 250 kilopascals | Review binder dosage and moisture |
| Loss on ignition and ash | Ignition at 850 degrees Celsius | Below 4 percent for reclaim | Investigate residue and fines |
| Grain distribution and fineness | Sieve analysis and AFS number | AFS 40 to 60 for grey iron | Adjust screen cut size or crusher |
Testing frequency should follow the risk. At least one set per shift for permeability and green strength is appropriate, because both respond within hours to changes in binder demand, while loss on ignition and acid demand can be checked weekly. Data collected only occasionally cannot be used for control. Trend the results against the sand-to-metal ratio, since rising loss on ignition precedes falling permeability.
The sand-to-metal ratio is the clearest single measure of how well a foundry manages its sand, expressing the tonnes of sand consumed for each tonne of metal poured. It is not a fixed target; it depends on alloy, casting weight, mould size, core content and machinery. What matters is that the ratio is known, broken down by line, and that changes are explained. An unexplained rise usually points to excessive dust carry-over or a dirty magnet.
| Sand system | Indicative sand-to-metal ratio | Main driver | Reclaim approach |
|---|---|---|---|
| Green sand with recycling | 5 to 15 | Batch size and shakeout practice | Scrub and magnetic separation |
| Resin sand, full reclaim | 10 to 25 | Binder cost and sand quality | Thermal regeneration plus attrition |
| Resin sand, partial reclaim | 20 to 40 | Core content and casting weight | Screening, magnet, dust removal |
| Large castings, single use | 40 to 100 | Casting mass and sand transport | Reuse in non critical moulds only |
Investing in reclamation is a calculation, not a slogan. Compare the cost of preparing one tonne of reclaim with the cost of delivering one tonne of new sand including transport, plus the disposal cost avoided on the spent fraction. Where reclaim is inconsistent, the hidden cost appears as extra binder, extra shakeout energy and more rejects. Track the disposal route cost as well, because landfill and incineration charges can change the answer.
The fraction of the sand stream that cannot be reclaimed still has to be handled. It typically contains the fine fraction, metallic fines, spent binder and any inorganic salt or additive used in the system. The first question is whether the material can return to the sand system. Where the foundry uses an inorganic salt binder, the spent sand is often usable as fill or as a neutralising agent elsewhere, subject to local rules and a leaching check.
Segregate the streams. Metal rich fines are collected separately from the dust collector and recycled into the metallurgical cycle or sold as a metal bearing residue, never mixed into fill material. Fine silica dust is a respirable crystalline silica hazard, so it must be handled under local rules for silica exposure, with sealed transfer, suitable protection and a documented exposure assessment. Keep records of the origin and date of each waste stream, its classification, the weight delivered and the site documentation.
There is no universal percentage. For resin sand many foundries start between 20 and 40 percent and move up as the reclaim proves consistent, while green sand systems run much higher. Set the blend limit from the reclaim's own test results, especially loss on ignition, permeability and grading.
The usual approach is loss on ignition, where a dried sample of used casting sand is burned at about 850 degrees Celsius and the mass difference is expressed as a percentage, with an ash figure indicating inorganic carry-over. Inorganic salt systems are assessed by chloride and conductivity of an extract.
Almost always excess fine material carried into the reclaim, because the dust collector is not removing enough or the crusher is generating fines faster than they are extracted. Check the filter differential pressure log, confirm the cyclone inlet is not bypassing, and compare a sieve analysis of the reclaim with virgin sand.
Not without checking local rules, because classification depends on the binder system and on additives such as sulphur, phenol or inorganic salts. In most jurisdictions the fine, metal bearing fraction is classified more strictly than the coarse sand. Obtain a written classification for each stream, keep the test data, and use a licensed receiver.
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