788 CHLORINATION 



consequence of the introduction of this process, the mines, after having been abandoned 

 for more than five centuries, were re-opened and worked with profit. The ore is first 

 roasted in a reverberatory furnace, and the arsenic thus becomes converted into 

 arsenious acid, which sublimes, and is condensed in a large chamber. The residue, 

 consisting chiefly of iron, is subjected to the action of chlorine, and the iron, with the 

 gold which it contains, then passes into the state of chloride; the chlorides are 

 dissolved out by water, and the gold precipitated by sulphuretted hydrogen, the 

 precipitation of the iron being prevented by previously acidifying the solution with 

 hydrochloric acid. The sulphide of gold is readily decomposed by 'heat, yielding 

 metallic gold on simple ignition. 



Since the introduction of the chlorination-process at Keichenstein, it has been 

 applied elsewhere with considerable success. At Schemnitz, in Hungary, the tailings 

 left after extraction of silver by Ziervogel's method, are treated with chlorine, and tne 

 gold, though amounting to only about 0'012 per cent, of the tailings, is thus profitably 

 .separated. In California the chlorination-process was first introduced, in 1858, by 

 Mr. Doetken ; and several chlorination-establishmente have since been erected in the 

 neighbourhood of Grass Valley. The process, as conducted in California, has been 

 described by Mr. Kustel. 1 



If the ore to be chlorinated contains finely-divided gold in quartz, it needs no pre- 

 paratory treatment beyond that of pulverization. But if sulphides and arsenides are 

 to be treated, they require to be previously roasted. The concentrated ores should be 

 roasted while damp, but pan-tailings should be first dried and then finely pulverized. 

 The roasting is most advantageously effected in a reverberatory furnace in which the 

 flame of the fuel is allowed to come into direct contact with the ores. A low tempera- 

 ture should be employed at first, and when much of the sulphur and arsenic is 

 expelled the heat should be gradually raised until any sulphates or arsenites which 

 are formed may be decomposed, and all the metals converted into oxides, excepting the 

 gold, which is set free in a metallic state. The lose of gold during roasting, though 

 stated by some to be considerable, seems really to be insignificant. 



In some cases it is desirable to add common salt (chloride of sodium) to the ore 

 during the roasting. If lead and antimony are present, no salt should be used, but if 

 the ore be associated with minorals containing lime, magnesia, or baryta, the addition 

 of salt is desirable. About 5 per cent., or 100 Ibs. of salt to the ton of sulphides, will 

 be found sufficient. The salt should be thoroughly incorporated with the ore, so as 

 to favour separation of the gold. It appears that the gold first forms a terchloride 

 (AuCP), which is converted at a higher temperature into protochloride (AuCl), and 

 this at a red heat, is reduced to metallic gold. 



The roasted ore, before being treated with chlorine, must be slightly moistened ; 

 this is effected by spreading the charge over the floor of an iron chamber, and 

 sprinkling it with water by means of a hose ; the ore being well stirred so that the 

 wet stuff is well mixed with the dry. After being sifted, to remove lumps, the ore is 

 .transferred to the chlorination-vats. These are large circular wooden vessels, each 

 capable of holding two or three tons of roasted ore. The inside of the vat is coated 

 with a mixture of pitch and tar, or with some similar substance, to prevent absorption 

 of the gold-solution by the wood. At a height of about one inch from the bottom of 

 the vessel, is fixed a false bottom, formed of boards laid together, with spaces be- 

 tween, and perforated with holes for the passage of the chlorine gas. A layer of 

 fragments of quartz, or other rock, is first spread over the false bottom, and the 

 coarser fragments covered by finer and finer layers till a stratum of fine sand is 

 placed on the top. The charge is laid on this prepared bed, which serves as a filter, 

 and chlorine gas is introduced through the false bottom. The gas is generated in a 

 leaden vessel. Sufficient chlorine for treating a charge of three tons of roasted sul- 

 phides may be obtained, according to Kustel, from the following mixture : Pulverized 

 manganese (peroxide) 30 Ibs., common salt, 30 or 40 Ibs., oil of vitriol of 66 degrees, 

 75 Ibs., and water, 45 Ibs. Having introduced the water, salt, and manganese into the 

 generator, the vessel should be covered air tight ; the acid is then gradually poured 

 in through a funnel, and the chlorine thus produced is washed by passing through a 

 vessel containing water to absorb any hydrochloric acid, the presence of which is 

 prejudicial. The evolution of chlorine is assisted by heat applied under the gas- 

 generator. When the chlorine, which passes from the wash-bottle to the chlorine- 

 vat, has permeated through the charge, the cover of the vat is luted on. The operator 

 should carefully avoid inhaling the chlorine, as this is a highly poisonous gas, and 

 extremely irritating when inhaled even in small quantity. 



The chlorine is allowed to act upon the ore for from 12 to 18 hours ; the coyer is 

 then removed from the vat, and a stream of water introduced. The solution of 



1 'A Treatise on Concentration of all kinds of Ores, including the Chlorination Process.' By 

 Gturlo Kustel. San Francisco : 1868. 



