In the assay of gold and silver in cyanide solutions the degree of accuracy and the speed desired are the governing factors in the choice of methods used and the quantity of solution taken for the determination.

Evaporation {Litharge) Method

To an evaporating dish add about 50 grams litharge and 146 to 292 cc cyanide solution. Evaporate to dryness, adding about 10 grams litharge during the evaporation. Scrape out the dried cake, and swab the dish thoroughly with a filter paper moistened with dilute HCl. Flux the cake and paper in a crucible, and cupel the resulting button. This method may be used on either pregnant or barren solutions and is accurate but requires considerable time to complete.

Evaporation (Lead Boat) Method

Evaporate sufficient cyanide solution to dryness in a lead-foil boat. When completed, fold the boat inward, hammer into a cube, and cupel.
This method is accurate and can be used for both pregnant and barren solutions.

Copper Sulphate Method

To 146 to 292 cc cyanide solution add 5 drops of saturated potassium ferrocyanide solution, then 15 cc of precipitating solution, and stir well. Carefully add 20 co H2SO4, and stir. Filter, wash, and dry the resulting precipitate. Mix the precipitate with the usual assay flux, add the filter paper to the crucible, and cupel the resulting button. This is a generally satisfactory method for both pregnant and barren solutions.

Precipitating Solution

The precipitating solution is made as follows: To a saturated solution of copper sulphate add a saturated solution of caustic soda until a light bluish-white heavy precipitate is formed. Add a little more caustic until the color changes to a darker blue, a heavy precipitate remaining. To this add a saturated solution of sodium cyanide until the precipitate is just dissolved and a yellow or light brown solution results.

Chiddy Method

To 146 to 584 cc (5 to 20 assay tons) cyanide solution add sufficient sodium cyanide, NaCN, to bring the strength to 0.50 per cent NaCN. Add 40 to 50 cc saturated lead acetate, Pb(C2H3O2)2, solution and then 5.0 grams zinc dust, stir well, and heat to boiling. Add 25 cc HCl, and allow to stand on a hot plate until the zinc is dissolved and a sponge forms. Decant the solution, and wash the sponge with water. Squeeze out the excess water, then dry the sponge, wrap in lead foil to bring the weight to about 20 grams, and cupel.

NOTE: at least 12 grams of sponge lead should be formed; otherwise the assays are usually low.
This method is usually found to be a satisfactory one for routine work, especially on barrens and lower grade solutions.

Color Test for Barren Solutions

To 1000 cc. of barren solution add
10 cc. saturated sodium cyanide, NaCN, solution.
3 chops saturated lead acetate, Pb(C2H3O2)2, solution.
2 grams zinc dust.

Shake well for several minutes. Transfer the sponge to a porcelain casserole, and decant the liquid. Dissolve the sponge in 10 cc aqua regia (3HCl + 1HNO3), and evaporate almost to dryness. Add 5 cc HCl, and evaporate to about 2 cc. Transfer to a test tube, cool, and add carefully 4 drops of saturated stannous chloride, SnCl2, solution. Note the color of the ring formed, and on shake-up note the color of the solution.
The following will serve as a guide to the relationship between color and value in cents per ton of the solution.

Faint pink (indefinite)……………………………………2
Faint blue…………………………………………………….4
Light purple………………………………………………….6
Deep purple………………………………………………….8
Deep purple with precipitate………………………….10
Black…………………………………………………..Over 10

It is advisable to make color tests on known barren solutions and establish a color scale to meet the individual mill solution requirements.

STANNOUS CHLORIDE (SnCl2) SOLUTION

To a 5 per cent HCl solution add stannous chloride until saturated. A few shavings of metallic tin in the solution will keep it in the stannous state.

Color Test for Silver

A few drops of a 10 per cent solution of sodium sulphide added to 25 or 50 cc of the plant solution to be tested give a pure white precipitate of zinc sulphide in the absence of silver. The precipitate becomes brownish in the presence of silver, and the depth of color is a close indication of the amount present. The presence of the usual small amounts of lead in the solutions does not affect the result appreciably.

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