Fluxing the Charge

The flux ordinarily used will vary with the composition of the precipitate and in order to determine the best mixture a number of small fusions may be made with varying proportions and the results noted. Borax is the most generally useful reagent but if used in large quantity, it makes the fusion rather expensive. Its place may be taken to some extent by carbonate of soda and sand, utilizing the tendency of bases to form fusible double silicates. For this purpose only sufficient soda should be used to form the second base in the double silicate. When the solutions are not properly filtered before precipitation there may be a considerable amount of silica in the precipitate and it may not be necessary to add any sand, but such a state of things usually implies bad working conditions. A little fluor-spar is sometimes used to flux difficultly fusible substances. For soda most text books recommend the bi-carbonate though for what reason it is difficult to see, and the writer prefers the normal carbonate in the form of commercial soda ash. Sodium bi-carbonate evolves a large volume of carbonic acid gas at a temperature below that of sintering of the charge, and if the precipitate is too dry, a considerable loss by dusting may occur; moreover, soda ash is less bulky and also cheaper per unit of fluxing power.

Julian and Smart1 give the following as a typical flux mixture. If sodium carbonate had been used instead of the bi-carbonate only about % of the above amounts of soda would have been needed.

By adding an oxidizing agent to the flux it is often possible to raise the fineness of the bullion by oxidizing some of the base metal and throwing it into the slag. Nitre is sometimes used for this purpose but it has a low efficiency owing to the rapidity of its reaction and E. H. Johnson and W. A. Caldecott3 have substituted manganese dioxide, and suggest the following mixtures:
Precipitate 100 parts
Fused borax 20 to 35 parts
Manganese dioxide , 20 to 40 parts
Sand 15 to 40 parts

They state that manganese dioxide has a tendency to carry silver into the slag, so if much silver is present, it should be used cautiously or not at all.

When an oxidizing flux is used the operation should be carried out in crucibles with removable or permanent fire-clay linings, as graphite would tend to counteract the effect of the oxidizing agent.

To one accustomed to the high-grade precipitates produced in the silver mills of Mexico the proportions of flux previously mentioned will seem excessive, and in such cases the quantity may be considerably reduced. A typical flux mixture for these rich silver precipitates is that used by Walter Neal1 at the Dos Estrellas mine in Mexico.

Precipitate 100 parts
Borax 15 parts
Bi-carbonate of soda 8 parts
Sand 4 parts
Scrap wrought iron in excess The iron is used to remove the silver from the matte which is so often produced in melting these precipitates even when no acid treatment is given. The same writer, in cleaning-up, makes two short-zinc products. He first screens his material with a 20- mesh sieve and afterward with a 60-mesh. The zinc remaining on 20 he returns to the boxes and that passing 20 but remaining on 60 he melts separately with a special flux: Short zinc 100 parts
Borax 40 parts
Soda 20 parts
Sand 10 parts
Lime 5 parts

The resulting metal, which contains 20% of zinc, is added to the high-grade bullion when the latter is remelted for running into bars.

A flux the writer has often found useful for clean silver precipitates consists of Precipitate 100 parts
Borax 15 parts
Sand 8 parts

Preparation of The Charge. Opinions vary as to the degree of mixture necessary. Some metallurgists break up the lumps of precipitate and pass the whole through a fine sieve, varying from Y in. to 20 mesh, before or during the addition of the fluxes, making a thorough mixture before charging into the crucibles, while others, avoiding breaking the lumps as far as possible, spread the precipitate in a layer on the fluxing table, sprinkle the fluxes evenly on top and shovel at once into the pot. The latter method has the advantage of reducing the dust loss to a minimum, but on the other hand is probably more suitable for a pure high-grade precipitate than for a base and dirty one, and the choice of procedure must be determined by the circumstances of each mill.

Briquetting has much to recommend it in the avoidance of accidental spilling of product outside the edges of the pot and minimizing of dust losses in charging, especially where it is the custom to add fresh batches of the material on top of a charge already partly fused, but it is expensive and also conducive to losses due to the additional handling necessary. Soda (Na2CO3 ) 8 parts

Charging. The practice in some mills is to charge a crucible only at the beginning of each fusion and to pour it as soon as ebullition ceases. It is a wise precaution against dust loss but is less efficient in fuel, time and labor, and its adoption should be determined by the value of the precipitate per unit of weight. To avoitf spilling when charging, the fluxed material is sometimes put into stout paper bags holding from one to two pounds, which can be placed in the crucible with a pair of tongs.