All-Fire Method of Gold & Silver Assay

All-Fire Method of Gold & Silver Assay

As this particular product holds a place by itself, it seems desirable to give a paper dealing especially with it as a corollary to my paper entitled “ The Litharge Process of Assaying Copper-Bearing Ores and Products, and the Method of Calculating Charges.”

The process is first to convert the metallic copper into matte by the addition of sulphur, allowing the two elements to combine at the bottom of the crucible when heat is applied. The flux then acts on the sulphides, oxidizing some of the copper which goes into the slag, while the gold and silver are collected in the lead-button reduced by the sulphur. These buttons are combined and scorified twice, for the purpose of concentrating the values and eliminating the remaining copper, thus reducing cupel absorption-loss to a minimum.

assaying-charge-for-a-20-gm-crucible

Method of Operation

Weigh out 0.25 A.T. of copper borings, divide it approximately into 3 equal parts, and place in 20-gm. crucibles; repeat the operation until four sets have been weighed out, thus having twelve crucibles in all for one assay. Weigh 800 mg. of pulverized sulphur into each of these, and mix with the copper; then add one charge of flux, but do not mix the copper and sulphur with the flux, as these two elements should remain at the bottom of the crucible, to form matte when heat is applied. Shake down; fill the remaining portion of the crucible with salt (NaCl), and place in a dull-red muffle. Raise the temperature very gradually for thirty or thirty-five minutes, at the end of which time some salt should remain, not quite molten, in the center of the top of the charge; this will melt and become quite fluid in a few minutes. The temperature should then be raised, so that, in forty-five minutes from charging, the muffle will be of a brigt-red color, the charge quiet, and perfectly fused.

The success or failure of this method, especially the silver result, depends upon the proper regulation of the furnace. Therefore, a detailed description of the manipulation, as practiced by the writer, is necessary at this point.

The muffle used is 17 x 19 x 8½ inches, outside dimensions, in which twenty-five crucibles can be placed at a time. Twelve crucibles containing the blister-copper charges should be placed in the front part of the muffle, so that the action can be watched carefully. In the back part of the muffle may be placed ore-assays, etc., the results of which are not so easily affected by temperature.

The atmosphere in the muffle must be reducing; otherwise, as the charge fuses, the silver seems to come to the surface, and a portion of it is apparently oxidized (or volatilized) and lost in the slag, making the result from 0.4 to 1.0 oz. per ton too low. A muffle that has a good draught through it always has an oxidizing atmosphere when fired with coal; therefore, some artificial means must be resorted to in order to bring about the desired result. The practice here is to plug the holes in the back of the muffle with bone-ash. Then distribute five crucibles (which are about three-fourths full of fine coal and covered with 3-in. scorifiers) amongst the charge, thus: Two in the back row, one in the center and two in the front row of crucibles, closing the front with a tight-fitting door. This will reduce 1 to 2 gm. of Pb from PbO, if a blank charge is run. In case a gas or gasoline furnace is used, the atmospheric conditions would probably be correct without resorting to artificial means.

Pouring, Slags and Buttons

All conditions being perfect, the charge will pour very fluid. But care must be taken to rotate the crucible quickly, and tap sharply several times, in order to settle any fine shots of lead that may otherwise be held in suspension or adhere to small recesses in the walls or bottom of the crucible.

The slag, on cooling, should be a yellow silicate of lead at the outside of the cone, becoming finely crystalline and deep- green immediately inside the outer skin. If more than merely the skin of the slag shows as a silicate, the heat has been too great; and if the entire slag is crystalline, with large coarse crystals pointing towards the center, the temperature has been too low, and has most probably left some shots of lead in the crucible. The salt on the top of the cone will be of a deep brownish-red, the depth of color lessening when overheated or when the atmosphere of the muffle had an oxidizing effect.

The button from each crucible should weigh about 18 gm., and break clean and bright from the slag. Care must be taken that a film of lead is not left on the slag where the button breaks from it, as a gas-bubble that has a thin covering of lead appears to form at the top of the button, which adheres to the slag.

Scorification

Each set is now represented by three buttons weighing 18 gm. each. These are now scorified, to eliminate more copper and concentrate the values. The four sets, each representing 0.25 A.T.,are manipulated as follows:

Place four 3-in. scorifiers in a hot muffle for ten or fifteen minutes; then, having reduced the temperature in the muffle to about the right heat to open cupellations quickly, place the sets of three each in the four scorifiers; close the door, and the scorifications should open as quickly as cupellations. When properly opened, lower the temperature to a heat that will just permit the operation to be carried on successfully until covered. Raise the temperature until the slag is hot enough to pour freely; decant as much slag as possible without losing any lead; return the scorifiers to the muffle and allow scorification to go on until covered again; close the door to heat up the slag, and then remove the scorifiers and pour. Each resulting button will weigh about 5 or 6 gm., if the temperature has been kept low from the start.

Second Scorification

Four 2-inch scorifiers are now heated as before. The buttons from the first scorification are broken down and the slags scraped free of any lead-films onto a filter-paper. Each 5-gm. button is made up to 25 gm. with C.P. test-lead. The filter-paper containing the button and test-lead is folded and put into the already hot scorifier. Conduct the operations at as low a temperature as possible, as these buttons will open easier than the first scorification. Raise the temperature when the buttons are nearly covered with slag, pour, cool, and break down again, watching for small amounts of lead on the slag.

These buttons are now ready for cupellation, and represent four assays of 0.25 A.T. each. Carry on this operation to get “ feather ” PbO on the cupels. This needs, careful watching, as the buttons only weigh 5 or 6 gm. Weigh the beads for silver plus gold in 0.25 A.T. Combine two and two, part, and weigh for gold in 0.5 A.T.

Summary

  1. The litharge must be absolutely free from silver, or, at the most, contain only small traces of silver, in order to avoid the necessity of correcting by difference. The kind used is of Pueblo manufacture.
  2. The temperature must be carefully regulated in all operations. Also, have a slightly reducing atmosphere in the crucible operation, otherwise the silver result will be too low.
  3. Great care must be exercised in watching for shots and films of lead, especially after the concentration of sets.
  4. The cupels should be nicely “ feathered,” and when cold should be of a very pale greenish-yellow, denoting the almost entire absence of copper. It is possible to get a good result by leaving more copper in the lead-button, thus doing away with the second scorification. It is best, however, to do as directed, placing beyond doubt the question of the absence of copper in the silver-beads.
  5. The flux can be mixed in large batches and measured by having a cup made that equals one charge, the formula being:
    method-for-the-assay-of-gold-and-silver
  6. The advantages of this method are:
    That it gives results in gold equal to the “ all-scorification ” method, and in silver equal to the combined wet and fire method.
    That it does away with the necessity of making separate estimations for gold and silver, thus saving the time and expense of working the combination-method for silver.
    That the time taken is less for each estimation than in the all-fire scorification. In practice it takes from five to six hours to do three determinations of gold and silver.