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
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
Several economic comparisons have been made between carbon systems and Merrill-Crowe systems. One of these, a systematic comparison completed by DeMent and King (1982) is summarized below.
Five separate conventional mills involving both Merrill-Crowe and the CIP processes were compared from an economic viewpoint. The gold concentration on the feed leach liquor ranged from 0.428 to 0.054 oz of gold per ton (14.6 to 1.8 g/mt) of solution.
Capital costs at the front end of a CIP plant are based on incoming flow rate, and are not a function of the gold grade. However, the capital costs of carbon stripping recovery portions do vary, as they are dependent on the grade of gold in the solution. Carbon regeneration is also dependent on the amount of carbon processed. On the other hand, with a Merrill-Crowe process, the total capital cost is solely a function of the initial flow rate.
Operating costs vary in the CIP plant according to the grade of gold in the leach liquor. This cost does not vary with the Merrill-Crowe except in the case of zinc usage which increases with higher input concentrations.
Gold losses in conventional CIP plants result from ….Read more
Gold mining uncovers more and more hydrothermal deposits in which gold mineralization is associated with sulfides and other compounds of base metals, arsenic, antimony, or tellurium. This type of mineralization frequently makes gold recovery difficult by conventional techniques, such as amalgamation, gravity separation, or direct cyanidation. Environmental aspects of processing these ores associated with arsenic or other toxic metals need to be carefully considered. The U.S. Bureau of Mines investigated an alkaline oxidative leaching procedure for treating gold-bearing arsenopyrite (FeAsS) ore, as one of the continuing efforts to develop efficient and environmentally safe metallurgical processes to enhance domestic mineral productivity and efficiency.
Since a significant portion of gold occurs in a submicroscopic form and possibly as a lattice constituent of the mineral, liberation of gold requires breakdown of the crystal structure of the host minerals by oxidation. (2) Oxidation by roasting is no longer an acceptable method for arsenic-bearing minerals, such as arsenopyrite, in the United States because of environmental regulations regarding arsenic emissions (<0.5 mg/m³ air, standard temperature and pressure). (3) Hydrometallurgical processes are likely to be used to treat gold-bearing arsenical sulfide minerals. Earlier practice in recovering gold from arseno-pyrite has been reviewed by the Bureau, and a ….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
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.
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
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
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.
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.