cyanide loss, then strengthened or standardized by the addition of cyanide, and, if needed, pumped back to the tub a. 28. Filters for Testing Plant. - A good filter bottom can be made for small testing plants by placing slats 1 inch apart across a hoop, as shown in Fig. 2 (b). These slats do not reach down the entire width of the hoop, but stop about 1 inch above the bottom to allow a free circulation of the liquor. A strip of canvas should be tacked between the circumference of the hoop and the inside of the tub to prevent sand from washing under the false bottom.

LABORATORY APPARATUS REQUIRED

29. The laboratory apparatus that will be required to make these tests are two burettes graduated to cubic centimeter, each having a capacity of 50 cubic centimeters and provided with floats; one burette stand; two pipettes having a capacity of 10 cubic centimeters; a graduated cylinder having a capacity of 1,000 cubic centimeters and a glass stopper; two titrating dishes having a capacity of 120 cubic centimeters; six porcelain evaporating dishes 5 inches in diameter; six iron evaporating dishes 5 inches in diameter; one wedge-wood mortar 5 inches in diameter; one iron stand with three rings; four flasks having a capacity of 8 ounces; and four flasks having a capacity of 16 ounces. A description of this apparatus is given in Assaying.

DETERMINATION OF FREE POTASSIUM CYANIDE IN A

SOLUTION

30. Silver Nitrate Test. - Several methods have been suggested for determining free potassium cyanide in a solution. A rapid and accurate determination may be made by titrating a measured quantity of a solution with a standard solution of silver nitrate. Silver cyanide is thus formed, which will immediately redissolve if there is an excess of potassium cyanide. The reaction is as follows:

(1)

(2)

(3)

AgNO, + KCN = AgCN + KNO,

AgCN+KCN=KAg(CN),

KAg(CN),+ AgNO, = 2AgCN+KNO,

31. End Point.-Titrating is finished when a permanent white precipitate of silver cyanide is produced. Silver nitrate is to be added from the burette until all the potassium cyanide has united with the silver cyanide to form a double salt of potassium silver cyanide. If more silver nitrate is added than is required to form the double salt, a permanent precipitate of silver cyanide is formed, which shows. that sufficient silver nitrate has been added for the reaction. The end reaction is more distinct when two or three drops of a 5-per-cent. solution of potassium iodide is added to the cyanide solution before titration. After all the cyanide is converted into the double salt, any excess of silver nitrate will unite with the iodide.

32. Preparation of the Standard Silver-Nitrate Solution.-A convenient standard for a silver-nitrate solution is a solution in which the quantity of silver nitrate in each cubic centimeter represents 0.1 per cent. of potassium cyanide. When such a solution is added to 10 c. c. of a cyanide solution, it will eventually form a permanent white precipitate. To prepare the standard silver nitrate, the equation of Art. 30 is taken; it shows that it takes two molecules of KCN to unite with one of AgNO,. The molecular weights are as follows:

AgNO, 108 +14+48 170

= =

From this it may be seen that the two are in the proportion of 17 to 13, so that if 17 grams of nitrate of silver be dissolved in 1,000 c. c. of pure water, 1 c. c. of the solution will be equal to .013 per cent. of KCN. This makes an unhandy number for quick calculations, therefore the nitrate of silver solution is reduced in proportion to 10 c. c. of KCN solution as follows:

13: 17: 10: 13.07

and

17: 13.07 :: 13: 10

That is, the standard silver-nitrate solution may be made by dissolving 13.07 grams of silver nitrate in 1,000 c. c. of distilled water.

33. To Calculate the Percentage of KCN in a Solution.—First fill two burettes, one with the standard silvernitrate solution and the other with the cyanide solution to be tested. Next, run into a beaker 10 c. c. of the cyanide solution; add cautiously the silver-nitrate solution from the burette until a permanent opalescent precipitate remains after thoroughly agitating the solution in the beaker. If 2 or 3 drops of a potassium-iodide solution be added to the cyanide solution in the beaker, it will assist in determining the end reaction by forming a yellow precipitate. Read from the burette the number of c. c. of standard silver nitrate used, divide by 10, and the result will be the available potassium cyanide in the solution in per cent.

To illustrate the above, suppose that 10 c. c. of the cyanide solution were used and that it took 6 c. c. of standard silvernitrate solution to form a permanent precipitate, then 1% = .6 per cent. KCN.

=

To test a

34. To Test Strong Cyanide Solutions. strong cyanide solution, take 3 or 4 c. c. and titrate it with silver-nitrate solution. If 4 c. c. of cyanide solution required 6 c. c. of silver nitrate, then 10 c. c. of cyanide would require 15 c. c. of standard silver nitrate; and 15 divided by 10 = 1.5 per cent. of potassium cyanide.

In testing the strength of a strong solution in the dissolving tank, take 10 c. c. of the solution and dilute it with water to 100 c. c. Take 10 c. c. of this solution and titrate with silver nitrate, as described above. The number of cubic centimeters required of the silver-nitrate solution will be the percentage of potassium cyanide in the strong

solution, for the 10 c. c. of the dilute solution only contained a tenth of the original cyanide solution, hence there is no need of dividing by 10.

35. To Test Very Weak Cyanide Solutions.-Take 100 c. c. of the cyanide solution, titrate it with the silvernitrate solution, and divide the number of c. c. of silver nitrate required by 100; the result will be the percentage of potassium cyanide in the solution. For example, suppose that 100 c. c. of a potassium cyanide solution be used and it required 5 c. c. of silver nitrate to form a permanent precipitate, then

5 100.05 per cent. KCN

36. To Test Potassium Cyanide of Commerce. Cyanide of potassium when purchased from the dealers is not pure. It may contain black carbide of iron, alkaline carbonates, and small quantities of alkaline chlorides and sulphides, for which reason it is customary to test the mixture to determine the quantity of KCN it contains.

In order to test the strength of solid potassium cyanide for the available KCN that it contains, proceed as follows: (1) Sample the cyanide to be tested by taking a specimen across the thickness of the cake. (2) Reduce the sample to a coarse powder, mix thoroughly, take a small sample of this and reduce to a very fine powder. (3) Weigh out 1 gram of the powdered sample and dissolve it in distilled water, after which add sufficient water to make 100 c. c of solution. (4) Take 10 c. c. of this solution and titrate it with standard silver nitrate. The number of c. c. of silver nirate required to produce the permanent precipitate divided by 10 will give the amount of available KCN in 1 gram of the salt. For illustration, suppose that 10 c. c. of the KCN solution required 8.5 c. c. of standard silver nitrate, then 8.5 10.85 KCN in 1 gram, which is equal to 85 per cent. of KCN in the crude salt.

ASSAY OF CYANIDE SOLUTION FOR GOLD

AND SILVER

37. Take 236.6 c. c., or pint, of the solution, place it in a round iron dish over a fire, and evaporate to a small bulk. As the evaporation proceeds rub the sides of the dish with a stirring rod so as to collect the salts in the bottom of the dish. To this concentrated solution add 40 grams of litharge. After mixing well, evaporate carefully to dryness. Transfer the contents of the dish to a clay crucible and add 15 grams of borax glass, 5 grams of bicarbonate of soda, and 2 grams of argol; mix the contents carefully with a spatula. Cover the contents of the crucible with a little borax glass and fuse. After quiet fusion pour the contents and allow to cool. Cupel the lead button and weigh the bead of gold and silver. If the ore contains silver, part the bead and weigh the gold.

38. Lead Evaporation Trays.-In some cyanide works the solutions are evaporated in trays made of thin sheet lead, which, with their contents, are afterwards scorified and the resulting lead button cupeled.

The lead tray, which should weigh about 20 grams, is made of pure lead foil and is shaped by laying a wooden block 2" x 4" on the foil and folding the lead upon the sides of the block so as to form a tray 4 inches long, 2 inches wide, and 1 inch deep. Care must be used in forming the corners of the tray so that they will not leak. For evaporation, the tray containing the solution is placed upon a piece of asbestos cardboard and heated gently by a burner underneath.

39. Gram and Grain Tables.-Consult the following tables to find the quantity of gold and silver per ton of solution. When the gold and silver are weighed with gram weights, refer to Table I; and when with grain weights, refer to Table II.