Free Cyanide vs Total Cyanide Determination

It is obviously of the highest importance in controlling the action of a solution to find out what strength of cyanide and alkali it contains at any given time. Under this head are the tests for free cyanide, “total” cyanide, hydrocyanic acid, and free or “protective” alkali.

Besides these cyanide constituents, which have to be determined frequently each day, it is useful at times to ascertain the presence, (and if present the amount), of the following substances, zinc, copper, ferrocyanide, and sulphocyanate.

It is customary to express cyanide strength in terms of potassium cyanide (KCN) of 100% strength, even though sodium cyanide (NaCN), carrying a higher percentage of cyanogen (CN) per unit of weight, may be actually in use.

This seems somewhat illogical and the only thing to be said in its defence is that the theory and practice of cyanidation having been born and having grown up under the formula KCN a change of notation would mean readjustment of all the old standards and, for a time at least, a mental translation of the new notation to the old before it would convey any meaning. In view of the varying cyanogen content per unit of weight in the commercial cyanide now put ….Read more

Cyanide Reflux Distillation Apparatus

Cyanide_Distillation_Apparatus Reflux distillation apparatus assembled as shown in Figure 1 or Figure 2. The boiling flask should be of 1 liter size with inlet tube and provision for condenser. The gas absorber may be a Fisher-Milligan scrubber.

Cyanide_distillation_Equipment Cyanide_Distillation_System

Distillation Procedure

Samples without sulfide. Place 500 mL of the combined sample or an aliquot diluted to 500 mL in the 1 liter boiling flask. Pipet 50 mL of 1.25 N sodium hydroxide into the absorbing tube. If the apparatus in Figure 1 is used, add distilled water until the spiral is covered. Connect the boiling flask, condenser, absorber and trap as shown in Figure 1 or Figure 2.

Start a slow stream of air entering the boiling flask by adjusting the vacuum source. Adjust the vacuum so that approximately two bubbles of air per second enter the boiling flask through the inlet tube. Proceed to Section 5.

Samples that contain sulfide. Place 500 mL of the combined sample or an aliquot diluted to 500 mL in the 1-liter boiling flask. Pipet 50 mL of 1.25 N sodium hydroxide ….Read more

Thiocyanate Assay Determination

Determination of Thiocyanate by Colorimetric

To a 100- cc Nessler tube add 50 cc water and 5 cc (more if necessary) of the cyanide solution to be tested, then add 2 cc HCl and 10 cc of 5 per cent solution of ferric chloride, FeCl3 Mix, and dilute to the 100-cc mark with water.

If Prussian blue should form on the addition of FeCl3, the solution should be filtered and the precipitate washed with water until all the red color is washed from the paper. Return the filtrate to the tube, and dilute to the 100-cc mark.

To a second Nessler tube add 50 cc water, 2 cc HCl and 10 cc 5 per cent FeCl3 solution. Then dilute with water to almost the 100-cc mark. From a burette run in 0.01 N potassium thiocyanate, KCNS, solution a few drops at a time until the color in tube 2 matches that in tube 1. When matched, read the burette, and calculate the amount of KCNS added to effect the match.

0.01N  KCNS SOLUTION. Dissolve 9.72 grams chemically, pure KCNS in water, and dilute to 1000 cc. This is 0.10N solution. Mix well, then take 100 ….Read more

Determination of Total Cyanide

Total cyanide is a term used to indicate, in terms of NaCN (or KCN), all the cyanogen existing in the form of simple cyanides, hydrocyanic acid, and the double cyanide of zinc.

Procedure. Measure 25 cc of clear cyanide solution, add 10 cc of caustic soda-potassium iodide solution, and titrate with standard AgNO3 solution to the first permanent yellow opalescence.
Standard AgNO3 solution (see free cyanide above)
Caustic Soda (NaOH)-Potassium Iodide (KI) solution. Dissolve 4 grams NaOH and 1 gram KI in 100 cc of water.

Determination of Total Cyanide by Distillation

Twenty-five cubic centimeters of cyanide solution is transferred to a 250-cc Claissen distilling flask leading to two 500-cc Erlenmeyer flasks connected in series. The first of these is placed in an ice bath. Each flask contains 150 cc of 2 per cent NaOH solution. Fifty cubic centimeters of dilute HCl (1 part 1.16 specific gravity acid to 4 parts water) is added to the Claissen flask through a separatory funnel. The contents of the Claissen flask are then boiled vigorously for about 6 min., after which the stopcock of the separatory funnel is opened to prevent “sucking back” and the flame removed. The contents of the Erlenmeyer flasks are then combined and agitated ….Read more

Free Cyanide Determination

You will perform this titration to obtain your Free Cyanide Determination.

  1. Take a 10ml aliquot of pregnant liquor.
  2. Make aliquot to 60ml with distilled water.
  3. Add 3 ml KI (10% solution)
  4. Add 5 ml of 1.5% NH4OH
  5. Titrate with 0.10 N Ag NO3 then; titre/100 = %CN


The endpoint is indicated by the first sign of a change from clear to slightly turbid conditions. A black background helps to perceive this transformation, and a comparison with a sample at equivalence point is also useful. If solution is already slightly turbid, a greed tinge will be noted at equivalence point.

Reagent Preparation:

Distilled water used at all times

Ag NO3        –                 originally N/50
–    dilute 1:1 to achieve N/100

10% KI                –    10 grams KI per 100 ml H20

1.5% NH4OH        –    1.5 mls Ammonia per 100 ml         H20

Ag NO3 should be kept in a dark colored bottle, away from light.

Standard silver nitrate solution is made by dissolving 4.33 grams silver nitrate, AgNO3, in distilled water and making up to 1 liter. The reaction between silver nitrate and sodium cyanide is represented by the following equation:

AgNO3 + 2NaCN = AgNa(CN)2 + NaNO3
Thus, 169.9 grams AgNO3 saturates 98 grams NaCN, or 4.33 grams AgNO3 saturates 2.5 grams ….Read more

Handling Waste Cyanide Solution

Tailings pulp carrying traces of cyanide and the discard of barren solutions in some cases constitutes a hazard to both humans and animals, and methods have been devised for destroying the contained cyanide.

A recent paper “The Treatment of Cyanide Wastes”by Chlorination by J. G. Dobson published in the Sewage Works J., November, 1947, discusses the subject as follows;

Free cyanide is one of the most toxic components of industrial wastes that are often discharged into sewers and streams. Because its toxicity to fish varies with pH, concentrations of other ions present, temperatures, and oxygen content of the receiving water—as well as activity of development and species of fauna in the steam—it is difficult to set up exact limits of concentration which can be safely discharged. Under some conditions, as little as 0.1 p.p.m. has proved fatal to fish. From 0.1 to 0.3 gram of CN is fatal to humans.

For 35 years or more various investigators have sought a satisfactory method of treating cyanide wastes. Among the methods that have been used are acidification and removal of the resulting HCN gas by air blowing, reaction with “lime sulphur,” aeration, treatment with ferrous sulfate, and oxidation with potassium permanganate.

The first of these methods ….Read more

Gold Chlorination Processes & Methods

In Liddell’s Handbook of Nonferrous Metallurgy, Vol. 2, 1945, there is to be found a very complete account of the uses of chlorine as applied to the recovery of gold and silver, in Chlorination Processes & Methods. Introducing the chapter entitled “Chlorine Metallurgical Processes” Liddell says:

Chlorine as a metallurgical agent appears to have lost ground during recent years, and much of what follows is of historical rather than operating interest. However, it is a question in the author’s mind whether the great decrease in the price of chlorine and the large sources of supply that will be available after the war do not warrant reinvestigation of the applicability of the chlorine process to present-day metallurgy. The use of chlorine can be broadly classified into the table of content here to the right:

Dry Chlorination

Chlorination Process for Gold

This process was based on the fact that chlorine, in the presence of moisture, converts gold into the trichloride AuCl3 , which is soluble in water and removed by washing, the gold being then precipitated by ferrous sulphate, sulphur dioxide, hydrogen sulphide, or charcoal. Coarse gold requires long contact and should be removed by amalgamation. Pyritic ore or concentrate requires a dead roast before chlorination; thoroughly ….Read more

Bromocyanide Process

Bromo salts are a mixture of 57 per cent sodium bromide, NaBr, and 43 per cent sodium bromate, NaBrO3, in the form of light-gray, light- yellow, or reddish-brown crystals or powder.

The use of bromo salts for treating a telluride concentrate in the Wright- Hargreaves plant at Kirkland Lake is described by J. T. Willey in E. and M.J., July 7, 1928.

Two 12- by 10-ft. tanks were used as collectors. They had mechanical agitators and a capacity of 15 tons solids and 15 tons solution. When thoroughly mixed, samples were taken for specific gravity, moisture, alkalinity, and cyanide content. If necessary, sulphuric acid was added to reduce alkalinity to 0.1 per cent, and cyanide was added to increase the strength to 1 lb. per ton.

Bromocyanide was next made by mixing bromo salts, cyanide, and sulphuric acid. To make 1 lb. bromocyanide it requires 521 grams bromo salts in 1500 cc water, 207 grams potassium cyanide in 1500 cc water, and 486 grams 66°Be. acid in 4500 cc water. Twenty pounds bromocyanide was made for each charge of 15 tons concentrate. The procedure was as follows:

Three 40-gal barrels were used. Into the first, which was immediately above the agitating tank, was put ….Read more

Cyanide Regeneration Processes and Methods

Cyanide regeneration offers a practical means of overcoming the otherwise heavy cyanide consumption frequently encountered in the treatment of gold and, especially, silver ores, where cyanicides (cyanide consuming minerals) are present.

Miscellaneous processes used for the extraction of gold and silver values by hydrometallurgical means are also discussed in this chapter. These include the carbon-cyanidation, the bromocyanide, the ammonia-cyanide, and chlorination processes. Though some of these are not used commercially today, they are of interest both historically and because they contribute to the sum of our general technical knowledge, out of which new and improved processes for the future may be developed.

A. J. Clark, metallurgist, once made the significant remark that every process, no matter to what extent it might be regarded as impractical, should be reviewed every 5 years with the thought in mind that improvements in equipment and techniques might possibly justify its revival as a working scheme.

In keeping with this viewpoint, we have included in the present chapter brief descriptions of certain processes generally, regarded as out of date and superseded by more modern and technically efficient methods, for the fact remains that in many cases the cost of treatment by our improved present-day metallurgy is often discouragingly high ….Read more

Carbon Cyanidation

Countercurrent carbon cyanidation with simultaneous dissolution of gold by cyanide and its adsorption by carbon offers several advantages over other carbon cyanidation processes as:

  1. the rate of dissolving the gold is faster,
  2. higher grade gold-bearing carbon is obtained,
  3. less carbon per ton of ore treated is required,
  4. separate dissolving and absorbing units are not needed,
  5. adsorption of dissolved gold by colloids or graphite is reduced, and
  6. capital and operating costs of plant are reduced.

Pilot-plant operations since 1940 which employed this method of cyanidation include one plant which operated without agitation and three plants which employed agitation but used different methods of separating the carbon from the ore pulp.

Carbon Cyanidation at the Harquahala Pilot Plant. The Eagle Picher Mining and Smelting Company in 1940-1941 erected and operated a 25-ton capacity pilot plant on Harqua Hala amalgamation tailing near Salome, Ariz. The tailing treated contained considerable colloidal material, and treatment of this material by standard cyanide and flotation methods had been tried without satisfactory results. The tailing was pulped with water, lime, cyanide, and activated carbon to a consistency of 70 per cent solids. The pulp was allowed to stand without agitation for a period of 15 to 24 hr., and then the ….Read more

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