PROCESS FOR PREPARING MOULDING SAND

The invention relates to a process for preparing moulding sand meant to be used especially for manufacturing cast iron moulds and moulding beds. According to the invention, the process consists in dosing 15...35 wt. percent of sand having a granulation greater than 0.3 mm in a proportion of at least 10% and containing 94% of SiO2 and at most 0.5% of K2O+Na2O, 80...85 wt. percent of sand having a granulation of 0.3...2 mm in a proportion of at most 45% and a granulation of 2...3 mm in a proportion of at least 55% and containing at least 98% of SiO2, 0.4% of Fe2O3,at most 0.4% of CaO+MgO, at most 1% Al2O3, 65...70% wt. percent of sand having a granulation under 0.3 mm, where the fraction 0.1...0.3 mm amounts to 70...75% with at most 1% of levigable component and containing at least 95% of SiO2, at most 1% of Fe2O3, at most 0.7% of K2O and Na2O and 85...100 wt. percent of sand having a granulation of 0.1...0.3 mm in a proportion of 80...85% and containing at least 96% of SiO2, at most 1% of K2O+Na2O, at most 1% of levigable component, afterwards the components being mixed in forced mixing mills for 5...10 min, the resulting sand having a granulation of 0...3 mm, and containing at least 88% of SiO2, at most 0.5% of K2O+Na2O, at most 1% CaO+MgO and at most 2% of levigable component, at a vitrification temperature of at least 1300°C. Inventia se refera la un procedeu de preparare a unor nisipuri metalurgice, destinate in special confectionarii formelor pentru lingotiere si poduri de turnare din fonta. Procedeul conform inventiei prevede dozarea, in procente de greutate, a 15...35% nisip, avand in proportie de minimum 10% granulatia de peste 0,3 mm si un continut de 94% SiO2 maximum 0,5% K2O + Na2O, a 80...85% nisip avand in proportie de maximum 45% granulatia de 0,3...2 mm si in proportie de minimum 55%, granulatia de2...3 mm si un continut de minimum 98% SiO2, 0,4% Fe2O3, maximum 0,4% CaO+MgO, maximum 1% Al2O3, a 65...70% nisip, avand granulatia sub 0,3 mm, din care fractia 0,1...0,3 mm are o pondere de 70...75% cu maximum 1% component levigabil si cuun continut de minimum 95% SiO2, maximum 1% Fe2O3, maximum 0,7% K2O si Na2O si a 85...100% nisip avand in proportie de 80...85% granulatia de 0,1...0,3 mm, si un continut de minimum 96% SiO2, maximum 1% K2O + Na2O, maximum 1% component levigabil, urmata de amestecarea componentelor in malaxoare cu amestec fortat timp de 5...10 min, nisipul rezultat avand granulatia de 0...3 mm, un continut de minimum 88% SiO2, maxi