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We have investigated the effect of addition of some of the inorganic and organic acids, in the naturally bonding sands taken from the bank of river Ravi, Lahore, Pakistan, on quality of casting and on mechanism of their reinforcement in the moulding sand. The test bars are cast in the moulding sands after adding little quantity (2%) of the following acids:FI3PO4, Tartaric acid (C4H606), Oxalic acid (H2C2042H20) and HCI. It has been found that phosphoric acid H3PO4 is the only acid which increases the mechanical resistance of the moulding sand in which it has been added and fulfills the requirements of grecnsand moulds which can be used for metal casting. The anions produced by the dissociation of phosphoric acid change the ions of 'first diffused layer' of clay to the normal ions of MC, CC, Kt, Na+ and hydrogen. Retaining samples in the air for 20- 30 min. largely improves dry compression strength, dry shear strength and resistance to torsion of the samples (see text). Studies with Scanning Electron Microscope have revealed that phosphoric acid dissociates into hydrogen and H2PO4- which develops cementitious compounds on self hardening. On the temp of the sand mould the conjugate H2PO4- further dissociates and phosphorous having high affinity for 02, oxidizes into the free oxides (see SEM images). Oxalic acid has the great capacity to leach iron oxide present in the clay at pH value 2.5-3 and temp. above 90°C. Tartaric acid do not have any significant effect because its conjugates do not impart any mechanical resistance to the moulding sand. HCI dissociates and Cl librated enters in the Skelton of the clay and reduces its mechanical strength. However author recommends that work on these acids addition in moulding sand, particularly the leaching effect of oxalic acid on the oxides of iron, present in the clays and silica sands, may be carried out further.
Pervaiz Habibullah, Zafar Abbas Hashmi. (2011) Study Cum Research on Effect of Addition of Organic and Inorganic Acids in Naturally Bonding Sand Used in Metal Casting, NUST Journal of Engineering Sciences , Volume 4, Issue 1.
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