COPPER ASSAYING 945 



porature is required. The button is then wrapped in paper with about an equal weight 

 of borax, and placed upon tho hot clay vessel. When the button is melted, the door 

 of the muffle is partly opened to allow tho air to enter, and the operations continued 

 until the copper brightens, appears of a green colour, and finally becomes solid. The clay 

 vessel is then removed from the muffle, and cooled by dipping it slowly in water. The 

 slag is then detached from the button, which should have the characteristic properties 

 of fine copper. The refining is also conducted on a cupel. Two cupels are treated in 

 a muffle, and upon one is placed the button of black copper with an equal weight or 

 more of lead, and upon the other the same weight of pure copper and lead. When 

 melted, air is admitted by partly removing the muffle-door, and the process continued 

 until the copper brightens. The buttons are then covered with charcoal-powder, re- 

 moved, and cooled in water, and weighed. The loss upon the pure copper is added on 

 to the assay button, and the result calculated. 



Wet Assay : 1. By Cyanide of Potassium. This is the best adapted and most direct 

 method for estimating copper in copper ores by means of standard solutions. It was 

 first proposed by Mr. H. Parkes. The process depends on the following reaction : when 

 a solution of cyanide of potassium is added to a blue ammoniacal solution of copper it 

 becomes colourless by the formation of a double cyanide of ammonia and copper. By 

 ascertaining by direct experiment the amount of cyanide of potassium required to dis- 

 charge the colour in an ammoniacal solution containing a given weight of copper, it is 

 easy by a comparative experiment to determine the amount of copper in a given weight 

 of ore. Tho standard solution of cyanide of potassium required, is prepared by dis- 

 solving about from 2,000 to 2,500 grains of cyanide of potassium (gold cyanide) in 4 

 pints (35,000 grains) of water. One thousand grains of this solution should be equal 

 to about 10 grains of copper. The solution is standardised as follows: A piece of 

 clean, electrotype copper, of from 4 to 7 grains, is accurately weighed, dissolved in a 

 small quantity of dilute nitric acid in a small flask, boiled to expel nitrous fumes, 

 and diluted with water to about half a pint. Ammonia is added in excess, when it will 

 become blue. A burette (for a description of this instrument, see ALKALIMETRY) is 

 filled to the top line with the solution of cyanide of potassium, and run in very slowly 

 to the cold blue solution of copper until it becomes nearly colourless, or of a faint 

 lilac tint. The number of divisions which are read off will be equal to the amount of 

 copper operated on. The experiment is repeated on a second or third piece of copper, 

 and tho mean result taken as standard. The decolorisation will require from ^ to f of 

 an hour, according to the amount of copper present. A piece of white paper should 

 bo placed under and behind the flask, to aid in observing the tint of the solution 

 during decolorisation. As the standard solution undergoes slow decomposition, it 

 will require correction about once a week. 



Process. From 10 to 50 grains of the ore are placed in a small flask or beaker glass, 

 and moistened with strong sulphuric acid ; nitric acid is then added from time to time, 

 and the whole digested at a gentle heat until nitrous acid fumes cease to be given off. 

 Water is cautiously added, and the solution reheated, to dissolve the basic sulphates 

 of copper and iron. When the ore is completely decomposed, the whole is transferred 

 into a pint flask and diluted to about ^ a pint ; ammonia is added in excess, and when 

 the solution is cold, the standard solution is gradually run in until there remains only 

 a faint lilac tint. The number of divisions used are road off, and the per-centage of 

 copper calculated. Example: 1,000 grains of the burette are equal to 10 grains of 

 copper, and 20 grains of the copper ore require 500 grains of solution ; therefore, 



1,000 : 500:: 10 :5and 



5x5 = 25 per cent, of copper. 



When peroxide of iron is present it should not be filtered off during tho assay, as it 

 would retain copper. Tho copper thus retained passes into solution during decolori- 

 sation. The alteration in the tint of the peroxide of iron will also afford a rough in- 

 dication of tho completion of the assay. The addition of sulphuric acid is only required 

 for the decomposition when sulphides are present in the ore. The presence of silver, 

 zinc, nickel, and cobalt, interfere with the estimation of copper by this method. Silver 

 is easily removed by adding hydrochloric acid or chloride of sodium, and filtering 

 before adding the ammonia. When zinc, nickel, and cobalt are present, the copper is 

 separated with the usual precautions by means of sulphuretted hydrogen, hyposulphite 

 of soda, or metallic iron, and the precipitates afterwards redissolved in acids, and the 

 solution assayed as before described. 



2. Precipitation Method. This is one of the oldest wet methods of estimating copper, 

 and when the requisite care is taken tho results are accurate. It is also useful when 

 burettes are not available. In some localities, and particularly in South America and 

 the United States of America, tho assay of copper ores is performed by this pro- 



VOL. I. 3 P 



