A method for producing of casting moulds by application of used foundry sand comprising at least the following steps: (a) providing a first mould material mixture comprising at least a first foundry sand and at least a binder based on water glass and a particulate metal oxide, wherein the particulate metal oxide is synthetically produced amorphous silicon dioxide, processing the mould material mixture and curing to a first casting mould, conducting a metal casting within the first casting mould so that a used casting mould and a casting is obtained and the used foundry sand is present in the form of a casting mould, (b) subjecting the used foundry sand, which is adhered with a binder based on water glass and with the particulate metal oxide added, to a thermal treatment, in which the used foundry sand is heated to a temperature of at least 200°C, wherein the thermal treatment is carried out until the acid consumption of the foundry sand, measured by the consumption of 0.1 n HCl in a 50 g quantity of foundry sand, has decreased by at least 10%, whereby regenerated foundry sand is obtained, and (c) producing a second mould material mixture by using a second foundry sand, wherein at least a part of the second foundry sand is from the regenerated foundry sand and wherein at least a binder based on water glass and a particulate metal oxide is added to the second foundry sand and wherein the particulate metal oxide is synthetically produced amorphous silicon dioxide, and curing of the second mould material mixture to a second casting mould in a tool, wherein the curing is carried by heating the tool containing the second mould material mixture to a temperature of 100 to 300°C or by blowing heated air having a temperature of 100 to 180°C into the tool. (a) providing a first mould material mixture comprising at least a first foundry sand and at least a binder based on water glass and a particulate metal oxide, wherein the particulate metal oxide is synthetically produced amorphous silicon dioxide, processing the mould material mixture and curing to a first casting mould, conducting a metal casting within the first casting mould so that a used casting mould and a casting is obtained and the used foundry sand is present in the form of a casting mould, (b) subjecting the used foundry sand, which is adhered with a binder based on water glass and with the particulate metal oxide added, to a thermal treatment, in which the used foundry sand is heated to a temperature of at least 200°C, wherein the thermal treatment is carried out until the acid consumption of the foundry sand, measured by the consumption of 0.1 n HCl in a 50 g quantity of foundry sand, has decreased by at least 10%, whereby regenerated foundry sand is obtained, and (c) producing a second mould material mixture by using a second foundry sand, wherein at least a part of the second foundry sand is from the regenerated foundry sand and wherein at least a binder based on water glass and a particulate metal oxide is added to the second foundry sand and wherein the particulate metal oxide is synthetically produced amorphous silicon dioxide, and curing of the second mould material mixture to a second casting mould in a tool, wherein the curing is carried by heating the tool containing the second mould material mixture to a temperature of 100 to 300°C or by blowing heated air having a temperature of 100 to 180°C into the tool.