Table of Contents
Gold & Silver Assay Method No. 1
Evaporate a known quantity in a square boat made of lead foil, scorify, and cupel. This is not reliable for low grade solutions because the amount that can be taken for assay is too small.
Gold & Silver Assay Method No. 2
Evaporate a known quantity in a large porcelain dish whose bottom is covered with a thin layer of litharge. When dry remove the litharge, carefully clean the dish with small pieces of filter paper dipped in dilute hydrochloric acid, the paper being dried, burnt, and added to the litharge. The latter is then fluxed and fused down in a crucible. The method may be used for amounts of solution up to 20 assay tons and is reliable though tedious.
Assay Method No. 3 of Gold & Silver (Virgoe)
To a known amount of solution add weak copper sulphate solution in slight excess, that is, until the clear filtrate shows a faint blue tint, then acidify with hydrochloric or sulphuric acid, and filter. It is also advisable to add a little sodium sulphite solution to ensure that the last traces of gold are thrown down. Should the solution be very weak in cyanide it is advisable to strengthen it to about 0.05% KCN before precipitating. For 10 assay tons of solution Clennell recommends
Copper sulphate (10% CuSO4.5H2O)……………………………20 cc
Sodium sulphite (15% Na2SO3)…………………………………..20 cc
Sulphuric acid (10% H2SO4)………………………………………10 cc
These are added in the order given stirring after each addition. The precipitate is then filtered, but washing is unnecessary.
When sufficiently drained 60 grams of the following flux are placed in the same filter paper:
Borax…………………………………30 parts
Litharge……………………………..30 parts
Charcoal………………………………1 part
The paper is then folded up and fused down in a small crucible, the resulting button of lead being cupelled.
Method to Assay for Au & Ag No. 4 (Chiddey-Magenau)
To 10 A.T. of solution (which if very weak in cyanide should be strengthened up to about 0.5% by addition of the proper quantity of concentrated solution) add 10 or 12 cc of a 10 per cent, solution of lead acetate and about 0.3 to 0.4 gram of zinc dust: stir and bring to a boil. Then add 15 cc of strong HCl and leave on the hot plate until the excess zinc has all dissolved, when the beaker should be removed and allowed to cool. Decant the solution and wash the spongy lead several times in large volumes of water. The lead may then be collected and pressed into a ball between the fingers. This ball is then folded up in lead foil in which a vent has been left for escape of steam, and cupelled.
Gold Extraction by Means of Cyanide of Potassium
Estimation of Gold in Cyanide Solution by Colorimetry
- Take approximately 1000 cc of solution in a tight stoppered jar. A Mason fruit jar or pickle bottle is convenient.
- Add 2 to 3 drops of lead acetate. (Clear saturated solution.)
- Add a pinch of zinc dust. (Approx. 2 grams.)
- Add 10 to 55 cc saturated NaCN solution to bring up to about 0.1 % KCN.
- Shake well for about 20 to 30 seconds.
- Pour out into a large evaporating dish.
- Settle and decant clear solution.
- Add 10 cc aqua regia and evaporate to 3 cc.
- Pour into small test tube and cool thoroughly. (Cooling is very important.)
- Add a few drops of fresh, saturated, stannous chloride solution.
The presence of gold will be indicated by a purplish ring or, if the tube is shaken, by a purplish tinge throughout.
The whole operation may be performed over a spirit lamp or similar flame and does not take over 5 minutes. The presence of as little as 0.02 dwt. gold per ton of solution is very plainly shown.
Colorimetric Estimation of Silver in Cyanide Solutions
A few drops of a 10% solution of sodium sulphide added to 25 to 50 cc of the solution to be tested, gives 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 very close indication of the amount present.