VALIDATION OF SFS RECLAIMED IN THE PROTOTYPE FOR MOULD MAKING
Evaluation of the use of reclaimed CHEM SFS in manufacturing chemically bonded moulds
Having in mind the sand mixtures used by ESTANDA in the facing part of the moulds for different series of casted components and, with the aim of evaluating the performance of sand reclaimed in the ECO-SANDFILL prototype as a substitute for thermally (TR) and primary mechanically (MR) recovered sand grades in those moulds, several formulations have been proposed in which different proportions of the three grades of sand are mixed together. Dosage of binders and catalyst is kept as in ESTANDA’s programmes for manufacturing mould facing sand (brake discs moulds and cement industry components moulds). Those formulations have been used to prepare standardised no-bake moulding sand specimens for mechanical tests to investigate the effects of reclaimed sand additions on the mechanical properties of no-bake resin bonded moulding sand.
The 8 formulations (PUNB sand mixtures) were characterised by ASK laboratory regarding flexural (transverse) strength (TS), scratch hardness (SH), strip time (ST) and bench life (BL). Among the various formulations tested, the formulation with sand mix S07 (75% TR + 25% ECO-SANDFILL) showed similar BL:ST ratio to the 100% TR sand formulation (S01) used as facing sand in moulds for brake discs (higher-end application). On the other hand, the TS curve of the formulation S05 (50% MR + 50% ECO-SANDFILL) coincided with TS curve for the formulation S04 (50% MR+ 50% TR) used as facing sand in moulds for cement industry components (medium-end application). Therefore, the sand reclaimed in the prototype appears as promising partial substitute for thermally recovered sand.
Validation of reclaimed sand as moulding sand in industrial casting trials
The re-usability in the foundry of the sand reclaimed in the prototype has been validated by using it as backing sand in moulds to manufacture series of steel parts in one of the industrial casting lines of the foundry. Besides, its potential for high-quality reuse as facing sand in moulds has been evaluated in two trials, during which moulds have been manufactured by hand (mixing, filling pattern flask and ramming of sand) and then incorporated into the industrial line for pattern removal and final steps in mould formation (painting, placing cores, assembling the two halves (coup and drag) and closing the mould) before pouring. Checks of scratch hardness on the moulds and of tensile strength of the moulding mixtures (specimens obtained during pattern filling) have been made, following the standard protocol in the foundry. Scratch hardness values measured are 69-70 in the scale hardness gauge, within the range 67-72 of the reference moulds; while flexural strength at 1 hr was 14-16 kg/cm2, below the reference value (20 kg/cm2) for sand mixtures with 100% thermally recovered sand.
The steel brake discs casted in those trial moulds have been examined to evaluate casting quality in comparison with other brake discs of the same pouring series. Quality control of castings in the foundry consists of two examinations, Visual Inspection and Magnetic Particles Testing. to detect surface and slightly subsurface flaws. Those quality checks are implemented on the parts once the raw castings have been subjected to the thermal treatment and have gone through shot blasting and rough machining steps, to clean them and improve their surface finish. Some porosity has been observed on the back of the discs, normal in these parts, and discs have been regarded as acceptable (ready for delivery to final client).
Besides, the brake discs casted in those trial moulds have been examined (macro and microscopic (SEM) inspections) by AZTERLAN. Surface defects observed (small cavities caused by mould-metal reaction, veining and drops due to mould material detachment) are not significantly different nor more frequent than defects occurring in the conventional parts that meet final customer’s specifications.
VALIDATION OF RECLAIMED SFS AS FINE AGGREGATES IN CONSTRUCTION APPLICATIONS
The concentration of potential hazardous substances in chemical composition and the environmental parameters in their eluates (leaching test) of the chemical (CHEM) SFS reclaimed in the prototype were below regulated limit values. Therefore, that sand could be safely used in bound (mortar, CLSM) and unbound applications (embankment) in the construction demos, provided that the corresponding technical requirements were also met. In this regard,
When defining SFS maximum substitution ratio achievable in bound applications (mortar and CLSM), different dosages of SFS (0%, 10%, 20%, 35%, 50%, 100%), have been studied and the performance (workability, durability, mechanical resistance…) of the mixtures evaluated. The evolution of mechanical strength over time is monitored through measurements of Uniaxial Compression Strength (UCS) at different ages along one year. Reclaimed CHEM SFS used in mortar mixtures with high natural sand substitution ratios (>50%) caused high cement consumption, growing water demand and a loss in mechanical properties. However, substitution ratios up to 35% can have a favorable effect on the mechanical properties since the fineness of SFS can cause a decrease in the porosity of the mortar and an increase in the density of the mixture (Figure 1). UCS values over 2 MPa were measured at 28 days for mixtures of CLSM with 100% substitution of natural sand with reclaimed CHEM SFS. It guarantees fulfilment of the target UCS >1 MPa set and, consequently, 100% substitution ratio would be used safely in that application.
In the case of the semi-dry screed mortars, the use of SFS (10% and 20% substitution) led to improved mechanical properties (flexural and compressive strength) of lab specimens cured at 65% and 100% relative humidity, comparing to the reference (0%). As a side effect, wet density grows with substitution ratio, due to variations in particle gradation.
Observed effects on increased flowability and water demand recommend arranging particle-size distribution carefully and replacing natural aggregates preferably in the size between 0.125-0.5 mm when preparing mortar mixtures with spent foundry sand.
That observation has been considered when manufacturing the two building mortar demos (self-levelling mortar for radiant floors and semi-dry screed mortars for outdoor use). Medium/long-term monitoring of those two mortar applications is underway, by visual inspection of trial slabs placed on site and by determining evolution over time of flexural and compressive strength values on test specimens cured under standard conditions (control experiments) and specimens cured under in-service conditions.