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Silver Production





Production

First stage of the silver production is ore enrichment - flotation and gravitational separation. Further extraction methods depend on the ore type and pyrometallurgical or hydrometallurgical. Polymetallic sulphide ores are not convertible by hydrometallurgical methods. For this reason they are first submitted to oxidizing and reducing or chlorinating roasting. During the lead ores roasting Ag2S is concentrated in Pb oxide and, then, in Pb metal. Two methods may be applied for lead ores processing. To recover silver from ores, the Parkes and Pattison method is used. In this method, the lead ore is heated with Zinc until it becomes molten. A method of separating lead from silver and to concentrate silver from a lead melt by the Pattison method, in which method a lead bath which contains silver is allowed to solidify partially, therewith to separate out the lead in a purer form, while the silver is concentrated in the molten residue. Subsequent to tapping of this residue, the residue is repeatedly subjected to the same process until a eutectic lead-silver alloy containing about 2.5% of silver is obtained, from which pure silver can be produced after expelling the lead. The purer lead crystals are melted and treated in a similar manner, so that after repeating the process a number of times, a lead free from silver is obtained.

Most of the silver produced worldwide from silver ores is extracted by the cyanidation process. The recovery of the precious metals involves two distinct operations - the oxidative dissolution of silver by an alkaline cyanide solution NaCN or KCN in Oxygen, and the reductive precipitation of metals from the solution by reducing or using anionites. Another method is amalgamation process of silver recovery in which the ore is passed over a surface of liquid Mercury and chlorides to form an amalgam that is subjected to fire-refining processes for the recovery of the silver.

Silver is also recovered during electrolytic refining in AgNO3 solution with silver precipitation on cathode is commercially applied. Fine silver contains at least 99.9% silver. Purities of 99.999 are available.

At the beginning, the manufacture of silver from its ores was developed in to three main divisions -
  1. amalgamation, employed in countries where fuel is scarce;
  2. lixiviation;
  3. smelting, the metal being subsequently separated from metallic lead or copper.


Silver Amalgamation

Various modifications of the amalgamation process have been employed in Mexico and Chile, but in recent years this method has been to a great extent supplanted by the cyanide process. Extraction by amalgamation is more difficult with silver than with gold. Mercury liberates silver rapidly from the chloride, bromise, and iodide, and very slowly from the sulphide. Other ores have to be converted before amalgamation into the chloride, effected by roasting with common salt, or by the action of common salt and copper compounds at the ordinary summer temperature.

In the patio process the finely ground ore is mixed in a patio or paved courtyard with mercury, common salt, and a mixture of copper and iron sulphates called magistral, prepared by roasting copper pyrites. The ore-heap or torta is kept moist. The reactions involved are obscure and complex, but it is supposed that some of them can be represented thus

Ag2S + CuCl2 = 2AgCl + CuS;
Ag2S + 2FeCl3 = 2AgCl + 2FeCl2 + S;
2Ag3AsS3 + 3CuCl2 = 6AgCl + 8CuS + As2S3;
2AgCl + 2Hg = 2HgCl + 2Ag.

The process of amalgamation requires from a fortnight to a month. The amalgam is decomposed by heating in retorts.

The pan-amalgamation process has found more favour than the patio process. The ore in the form of fine sludge is stirred in iron pans with a mixture of mercury, common salt, and cupric sulphate. When the action is complete, the excess of mercury is drained off, and the amalgam is allowed to settle, and then decomposed by heat. In the Boss system the process is continuous, a series of pans and settlers being employed. Some silver ores, notably those containing sulphides of arsenic, antimony, copper, iron, and zinc, are roasted with common salt before amalgamation.

Among the older methods is the cauldron or cazo process for ores free from sulphur. The ore was reduced by boiling with a solution of common salt in copper vessels, and then amalgamated. In the Franketina process sulphide ores were roasted with common salt, and then boiled with a solution of salt in presence of mercury in copper-bottomed vessels. In the Kronke process decomposition of the ore is effected by a hot solution of cuprous chloride and common salt, reduction to metallic silver and amalgamation being effected by addition of mercury and an amalgam of lead or zinc. In the obsolete Freiberg barrel process sulphide ores were roasted with salt, and amalgamated in rotating barrels with mercury, iron being added to prevent formation of mercury chlorides.

Silver Lixiviation

The silver is dissolved from the ore by an aqueous solution of a salt, and then precipitated as metal or sulphide. The cyanide process is the most important of the lixiviation methods, its application having been considerably extended in recent years, especially in Mexico. The ore is very finely crushed with cyanide solution in a stamp-mill, and the sludge produced submitted to agitation and aeration in contact with cyanide solution. The liquid is separated from the ore by the aid of mechanical filters, and the silver precipitated from the clear solution by addition of zinc in the form of dust or shavings. The product is smelted with nitre, and is sometimes refined by blowing air through the molten mass.

The bulk of the Mexican ore consists of argentite. The cyanide dissolves the silver sulphide

Ag2S + 4NaCN = 2NaAg(CN)2 + Na2S.

The aeration oxidizes sodium sulphide to thiosulphate and hydroxide, and ultimately to sulphate. A side-reaction is the formation of thiocyanate

Na2S + NaCN + H2O + O = NaCNS + 2NaOH.

The elimination of sodium sulphide is facilitated by addition of lead oxide or acetate or mercuric chloride, the corresponding sulphide being precipitated. During aeration, the lead sulphide is assumed to be converted into lead oxide, thus explaining the great effect produced by the presence of a very small percentage of lead salts

PbS + NaCN + O = NaCNS + PbO.

A combination of lixiviation and cyaniding has also been applied to certain Mexican ores.

The Ziervogel process can be worked with argentiferous copper mattes free from lead, arsenic, antimony, and bismuth. By roasting the matte in an oxidizing atmosphere, the iron is converted into sulphate. About 700° C. this substance is decomposed, the copper being converted into sulphate. At 840° to 850° C. the copper salt is converted into cupric oxide, and silver sulphate simultaneously formed. At this point the roasting is stopped, the silver sulphate is extracted with hot water, and the silver precipitated by means of metallic copper. These mattes are now usually worked for copper, and the silver separated electrolytically.

Other obsolete methods include the Augustin process, in which the sulphide ore was roasted with salt, and the silver chloride dissolved in hot brine; the Patera process, in which the brine was replaced by a solution of sodium thiosulphate; and the Kiso process, in which calcium thiosulphate was employed. In the Russell process the ordinary extraction with thiosulphate was followed by treatment with sodium copper thiosulphate, or " extra solution," the object being to extract any metallic silver and undecomposed sulphide present.

Silver Smelting

Smelting is applied to argentiferous lead and copper ores, the silver being concentrated in the lead or copper produced.

The pig lead contains about 2 per cent, of silver, and there are three processes for its desilverization, the final stage of each being cupellation. In the almost obsolete Pattinson process the lead is melted and then allowed to crystallize. The first fractions consist of almost pure lead, and the crystals are removed by means of perforated ladles. The crystallization is continued until the residual lead contains about 1 per cent, of silver. In the Rozan process the molten metal is agitated by revolving paddles or jets of steam, and the concentration continued up to 2 per cent. In the Parke process the silver is extracted from the lead by successive additions of zinc, the argentiferous zinc rising in crusts to the surface, and being ladled off. After liquation to remove some of the lead, the zinc is distilled from a retort, the residue consisting of lead and about 5 to 10 per cent, of silver. Where a demand for zinc sulphate exists, the zinc is also converted into this salt by oxidation with steam and solution in sulphuric acid.

Cupellation is effected by oxidizing the argentiferous lead in a reverberatory furnace with a hearth of bone-ash, marl, magnesia, or Portland cement and crushed fire-brick, the litharge formed being kept liquid by maintaining the temperature above 900° C. The litharge and the oxides of other base metals flow to the edge of the bath of molten metal, and are drawn off. By cupellation it is possible to obtain silver containing only 0.2 per cent, of impurities, but it has often to be cupelled again with more lead. Different types of cupellation-furnace are employed in Great Britain, the United States, and Germany.

In the electrolytic refining of copper both silver and gold are deposited in the insoluble sludge at the bottom of the vessel, and are subsequently extracted from this sludge, the silver being dissolved by boiling with sulphuric acid, and subsequently precipitated by copper.
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