Moulding sand, and a process for the preparation of moulding sand for the shell moulding process

摘要

Moulding sand for the shell-moulding process consists of grains which are coated with a binding agent consisting wholly or mainly of one or more lignin-novolak condensates. The binding agent may also contain a ketone-aldehyde synthetic resin. The lignin-novolak condensate may be prepared by the acid condensation with phenol and formaldehyde, of a reactive active lignin made by the saccharification of vegetable substances containing hemicellulose and cellulose by means of 35-41% hydrochloric acid at a temperature not exceeding 50 DEG C., or by replacing part of the phenol in novolaks with active lignin. Such a coated sand may be prepared by giving the sand a dust-like coating of the lignin novolak condensate, adding a small proportion of a liquid which will swell the condensate, and thereafter permitting the liquid to evaporate and thereby obtaining a dust-free, non-caking, coated sand. The liquid may be a mixture of a component that boils below 100 DEG C. and a component with a higher boiling point, for example between 150 and 220 DEG C., in which case the coated sand will contain some of the higher boiling point component. The liquid may also contain a synthetic resin, such as a ketone-formaldehyde resin, or contain constituents capable of forming a ketone-formaldehyde resin during the operation, for example acetone and hexa-methylene tetramine or paraformaldehyde. An alkaline-earth metal stearate, for example calcium stearate, may be incorporated in the coating to facilitate removal of the mould from a hot cast article. Specification 845,789 is referred to.