Ferrochrome binder, also known as "Versimould Binder", is an aluminium phosphate solution with excellent binding and refractory properties. Its bond is strongest with metals and alumina. Its properties are similar to sodium silicates, but the main difference is that it has high strength at high temperatures, whereas sodium silicates are molten glasses at high temperatures.
Ferrochrome binder may be used to bind ferrochrome fines to form ingot moulds. Ferrochrome fines are often used to form loose beddings for ferrochrome ingots. The reason for this is purely to prevent contamination. When ferrochrome binder is used, the function of the ferrochrome fines is to provide HIGH STRENGTH AND HIGH THERMAL CONDUCTIVITY, and has nothing to do with chemical compatibility. Such ingot moulds are equally suitable for any metal. Ferrochrome fines make the best agregate for such ingot moulds because of their high thermal conductivity and stability against volume changes due to oxidation. The surface of the ingot mould must be isolated from the molten metal. Graphite paint is usually the best coating because it allows excellent release of the ingot.
Ferrochrome binder develops increasing strength with increasing temperature. It undergoes a series of dehydration reactions, the last of which occurs at just below 400°C.
Preparation; Normal crusher fines have a distribution of particle sizes suitable for consolidation to a high density. Minus 3mm would be suitable. Mix the ferrochrome fines with sufficient ferrochrome binder to coat all the particles but not saturate the pores between the particles. This condition is achieved between roughly 3% and 20% binder by weight. 8 to 10 % is probably best in most gradings. Low additions give low strength and oversaturation will lead to blistering as the vapour will not be able to escape.
Compact the fines into the rough shape desired for the mould. Tramping it into shape with gumboots is fairly effective. The ideal means of compaction will depend on the grading of the fines. Scrape off the excess to get the final shape of the mould. Sharp corners are undesirable and a good taper is preferred. If the ingot is to be removed with a grab, gaps must be left and filled later with loose fines.
Drying; The mould must be dried to at least 400°C. A gas flame is ideal. Keep the burner on until the surface of the mould starts to glow.
Coating; Spray with graphite paint until the surface is smooth. The graphite paint may be applied while the mould is still hot. Re-spray the mould as soon as the aggregate can be seen or if a piece breaks off.
Tapping; Ideally the metal should flow slowly into the mould from a low angle runner or tundish. If the metal is squirting in from a height or under pressure, the angle of incidence should be kept low, or some metal scrap should be placed over the area of impact. In difficult applications, a striker block of Keraplas may be built in. If the graphite paint is consistently washed away, a coating of green refractory paint may be required.
Maintenance; The mould will inevitably crack, from various causes, and should be repaired with Keraplas. Cracks must be repaired before graphite paint is sprayed. If chunks break off, Keraplas repair may be too expensive, and repair may be done with ferrochrome fines, a few % Versimould hardener and perhaps a little clay or alumina. This forms a very rapid hardening highly exothermic compound which should be handled cautiously with rubber gloves. If the moulds give unsatisfactory lives, various steps may be taken, eg. use of supplementary binders, thickening of the walls, reinforcing, modification of the agregate grading, modifaction of the shape of the mould, improvement of ingot handling etc.