Cyanide Leaching

Thiourea Silver Leaching

Unlike base metals which are readily soluble in mineral acids, noble metals like gold and silver require the presence of oxidizing and complexing agents for dissolution. Silver approaches base metal behavior in acidic thiourea solutions (Ag + 3Thio ↔ Ag(Thio)3+ + e, E° = 0.025 V) requiring only mild oxidizing conditions for dissolution. Accordingly, the following reactions characterize the dissolution of silver in acidic thiourea with various oxidants


Ferric sulfate and chloride have been frequently employed as oxidants. Other oxidants that appear compatible with acidic thiourea systems include Na2O2, O3, KBrO3, and K2Cr2O7.

Like the metallic form, certain silver minerals require oxidizing conditions for dissolution in acidic solutions. Argentite (Ag2S) in the presence of thiourea is leached by ferric ions according to the following overall reaction


It is reasonable to believe that refractory silver minerals like Hessite (Ag2Te) would respond to acidic thiourea leaching similar to the chemistry represented in Eq. (6).

Cerargyrite (AgCl) is a silver mineral that will dissolve in acidified thiourea solutions under nonoxidizing conditions. This mineral proceeds to solution by simple complexation dissolution. The following reaction represents the dissolution of AgCl in thiourea

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Zinc Box Gold Precipitation

The simplest precipitation equipment for a small mine is what is known as a zinc-box (Fig. 150). This consists of a long, narrow, sloping, wood or painted sheet-iron box divided into wide compartments or cells which are separated by baffle-boards and narrow compartments. The bottom of the box slopes to one side to facilitate the clean-up through plug-holes which discharge into a small launder. Near the bottom of each large compartment is a ledge on which is supported a heavy wire screen-tray. These compartments are filled firmly yet are springy with zinc shavings or filament. The gold- and silver-bearing solution is turned into the first narrow compartment, and the baffle, which does not reach the bottom, deflects the solution under the tray holding the zinc. The solution flows upward through the zinc, over the side of the compartment, down by the next baffle to the next lot of zinc, and so on, through 6 to 10 cells to the discharge, whence the solution flows to a sump or storage-tank. At this point it should be practically barren of precious metals. One cubic foot of filament will serve 6 to 10 tons of solution a day.

From time to time during a ….Read more


As coarse gold is not dissolved by cyanide, it must be removed from an ore by one of the methods already described. Weak solutions of sodium cyanide (or potassium cyanide, seldom used) will dissolve the fine, untarnished gold and silver in ores and tailings, provided the latter have been ground fine enough and do not contain any material, such as acid compounds or minerals, that would affect the cyanide. The gold and silver are recovered or precipitated from the solutions by means of zinc shavings or zinc dust. Melting of the dried precipitate with fluxes yields marketable bullion. An entire ore may be slimed and cyanided, or it may be divided into sand and slime and then cyanided. Tailings may be sand or slime or a mixture of such. Barren solutions, when strengthened with more cyanide salt, can be used again. The strength of solutions must be known, but this is easily determined, as described later.


Two types of cyanide are used in treating gold and silver ores and tailings, also in the flotation of ores: sodium cyanide, which contains 98 per cent NaCN, and calcium cyanide (Aero brand), which contains up to 50 per cent NaCN equivalent. The wholesale price ….Read more

Precipitation of Copper contained Silver leached out with Copper

Having refractory ore under treatment, it is generally the case that copper is also found in it. While roasting, the presence of copper is favorable for the chlorination of the silver, but copper ores require some more salt, especially if it is intended to save the copper also. The more chloride of copper formed, the more will be found in the solution while leaching it with hot water. In order to convert all the copper into a chloride, it would take at least one and a half pounds of salt to each pound of copper; and considering other base metals, lime, etc., all of which absorb chlorine, while a considerable part escapes useless, the above quantity has to be doubled. For this reason no special attention can be paid to the copper ; only that part of it can be extracted which is converted into a chloride during roasting under the usual circumstances. The chloride of copper transfers a part of its chlorine to the silver and other metals (23), and is reduced thereby to a sub-chloride ; if there is sufficient salt in the furnace it is raised again to a chloride. This sub-chloride (Cu2 Cl) is not soluble ….Read more

Zinc Box Construction and Operation

The precipitation by zinc shavings is carried out in rectangular boxes or troughs, divided by transverse partitions into a number of compartments. The partitions are so arranged that the solution flows alternately downward through a narrow compartment, and upward through a wide one, the latter alone containing the zinc shavings. The height of every alternate partition is one or two inches lower than that of the succeeding one, so that the liquid, after ascending through a wide compartment, overflows the lower partition into the narrow space between it and the next (higher) partition; the lower edge of the latter is raised several inches above the floor of the box, so that the solution flows under it into the next wide compartment, and so on. (See Fig. 37.)

The dimensions of the boxes vary, according to the work required of them; the following may be taken as representing ordinary conditions: Length, 12 to 24 ft.; width, 1½ to 3 ft.; height, 2½ to 3 ft.

The sides, ends, and bottoms are usually constructed of 1½-in. boards, strongly held together by bolts passing horizontally from side to side of the box, generally through the narrow compartments. Vertical bolts are also used for securing the ….Read more


The study and use of ferrocyanides was initiated with the discovery of the pigment “Prussian Blue” by Diesbach in 1704. Thus, they are among the earliest commercial chemicals and have been produced in large quantities for many years. In the United States, American Cyanamid Company, with its ample supplies of cyanides as starting materials, has been the principal manufacturer of ferrocyanides.

Because of their use in the preparation of pigments and because of their unique properties, the ferrocyanides have been the subject of many investigations, and a large amount of information on them is available in the scientific literature. This book presents a summary of the known physical and chemical properties of sodium and potassium ferrocyanides and of hydroferrocyanic acid together with known and proposed applications. The field of pigments is so broad and so complex that a major part of the literature on ferrocyanides has been devoted to it. The purpose of this book, therefore, is to emphasize many of the applications that have been neglected previously. It is felt that the information presented in this review will be of value not only to a variety of different industries, but also to scientists in schools and colleges. The properties of ….Read more

Batch Cyanidation Leach of Flotation Concentrate


The production of gold bullion from gravity or flotation concentrates is often an important economic consideration for an isolated gold mining operation. It is assumed in this case that the coarse free gold has been recovered by the Mineral Jig in the grinding circuit and that the jig concentrate has been amalgamated. This treatment produces a portion of the gold (25 to 40 percent of the total) as amalgam, but the balance of the recoverable gold is present as non-amalgamable flotation concentrate together with the amalgamation residue. The problem is therefore, to develop a batch cyanidation procedure that will convert the gold contained in these products into bullion. A batch procedure is desired because of the small quantity of total concentrate (5 to 6 tons) produced daily and the greater flexibility and simplicity afforded by a batch type process.


The Flowsheet

Gravity and flotation concentrates that contain a portion of gold closely associated with sulphides may require fine grinding (—200 mesh) followed by pre-aeration and/or pre-agitation with lime prior to cyanidation in order to minimize the chemical consumption. Refractory gold concentrates may require roasting, but this ….Read more

Cyanide Destruction Hypochlorite / Chlorine

In the Cyanide Destruction by Hypochlorite reaction, the pH has a strong inverse effect on the ORP. Thus, wastewater treatment facilities must closely control the pH to achieve consistent ORP control, especially if they use hypochlorite as the oxidizing agent. Adding hypochlorite raises the pH, which, if unchecked, lowers the ORP. calling for additional hypochlorite. Controlling the pH at a setting above the pH level where hypochlorite has an influence and separating the ORP…

Alkaline Chlorine-Hypochlorite Oxidation: Chlorine was used for cyanide destruction in the early days of cyanidation in the late 1800s, because chlorine and its derivatives were readily available in the industry at that time. The method has been applied ever since in a variety of forms.

The active reagent for chlorine oxidation of free and complexed cyanide is the hypochlorite ion, produced when chlorine dissolves in water, as described. Alternatively, hypochlorite ions can be produced by dissolving suitable salts, such as sodium or calcium hypochlorite, in water.

Free cyanide reacts rapidly with hypochlorite (OCl) in aqueous solution to form cyanogen chloride, otherwise known as tear gas.

Cyanide also reacts rapidly with free chlorine.

However, at high pH, cyanogen chloride is readily hydrolyzed to cyanate and chloride ions.

In ….Read more

Extraction Of Silver

The extraction of silver by the solving processes simple. The ore is first roasted with salt in the usual way, whereby the formation of base metal chlorides cannot be avoided entirely. After roasting, the ore is first subjected to leaching with water, in order to extract the base metal chlorides, and then with hyposulphite of lime, to extract the silver.

Extraction of Silver

After a chloridizing roasting the ore should be examined to ascertain the amount of chloride of silver contained in it, according to 21. In case the extraction should not be satisfactory, it is then easier to find what the cause is. The ore is then prepared for leaching.

First Leaching

The roasted ore contains chloride of silver, which does not dissolve in water, but generally there are also base chlorides in it, as the chlorides of copper, zinc, lead, iron, antimony, etc., which are soluble. It is the purpose of the first leaching to extract these base metals by means of hot water. For this purpose the ore is introduced into a tub or square box of pine wood, the planks being one and one-half to two inches thick.

gold silver roasting false bottom….Read more

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