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Amalgamation - in Metallurgy


AMALGAMATION - IN METALLURGY Amalgamation is the process of extracting gold and silver from their ores by means of mercury. It depends on the fact that mer cury, or quicksilver as it is usually called by metallurgists, "wets" and adheres to metallic gold and silver and penetrates them, form ing pasty amalgams of a silvery colour. Particles of amalgam readily adhere to each other, and the aggregates become large and heavy enough to sink in running water which carries away sand and other earthy materials. Pieces of gold, if not too small, sink perfectly well of themselves and adhere to mercury or amalgam lying at the bottom of the stream, and gold ores have been widely amalgamated on this principle. Coarse gold can be saved without the aid of mercury but finely divided gold is more likely to escape. Grains of silver naturally settle also, but most silver ores do not contain metallic silver. They contain compounds such as silver sulphide (silver combined with sulphur), and the compounds will not settle in water and must be decomposed to set free the silver before it can be amalgamated.

In amalgamation, ores are crushed to pulp in water and mixed with mercury with violent agitation in order to break up the mer cury into minute globules and disperse them throughout the mass, and so enable them to be brought into contact with the gold or silver. Afterwards by dilution with water and gentle stirring, or by a running stream of water, the amalgam and surplus mercury are induced to coalesce and settle to the bottom, where they are caught in a pool of mercury or on the surface of copper plates to which they adhere. When the earthy matter has been washed away, the mixture of metals is put into filter bags and squeezed to remove the surplus mercury. The liquid mercury passes through, containing a minute quantity of gold and silver dissolved in it, and pasty amalgam containing from 25% to 5o% of gold remains in the bag. The amalgam is heated in retorts (fig. 5), the mercury driven off as vapour, only to be condensed in a cool chamber for use again, and the gold and silver now nearly pure are melted down and cast into bars. Since all gold ores contain some silver and silver ores some gold, the bars consist of a mixture of gold and silver which are afterwards parted (see GOLD). The process is a very old one. The extraction of gold by mercury is mentioned by Pliny in his Natural History, and descriptions of amalgamation processes for both gold and silver are to be found in various i6th century treatises. The history of the process is given in detail by Percy.

Mercury may, however, become so excessively subdivided, as the result of too violent agitation or pounding, especially when mixed with grease or certain kinds of ore, that it becomes "floured" or "sickened." Floured mercury is a white powder like flour, although under the microscope it is seen to consist of minute globules of apparently ordinary mercury, heaped together. The globules will-not coalesce and are carried away and lost in the tailings or refuse. They are generally separated only by films of air, and may be made to coalesce by the action of acids or in other ways entailing expense or loss. Again, although earthy matter and mercury have no action on each other, nevertheless mercury attacks and amalgamates copper, lead, zinc and some other metals, especially when they are present in the metallic state. The gam thus becomes debased and powdery so that part is lost and the remainder is costly to refine. There is also the case of ores which do not yield a reasonable percentage of their values when treated by amalgamation ; these are called refractory and those which are amenable to amalgamation are described as free-milling. Patio or Mexican Process.—Silverore was ground by mule power in arrastras or shallow circular pits paved with stone (fig. 1). Large blocks of stone attached by beams to a central rotating post were dragged round the arrastra and reduced the ore to fine mud which was taken out and spread over a courtyard or patio in low heaps. It is then kled with mercury and chemicals (common salt and copper phate) and mixed by mules which were driven over every part of the heaps. Complicated chemical reactions took place, never fully understood, the effect of which was to free the silver from its intractable compounds and enable it to be taken up by the mercury. The heaps were trodden by the mules every day or two until the amalgamation was complete, which might require a month. Lastly the material was agitated with water in large tubs and the mud run off through the plug holes. The amalgam found at the bottom was collected and treated as already described. The process was especially suitable for the silver ores of the dry barren districts of Mexico, where water and fuel are scarce, and was not finally displaced by the cyanide proc ess (q.v.) until early in the loth century. It was used in the pro duction of a large proportion of the world's silver for 35o years.

Pan Amalgamation Process.

Theprocess was used for sil ver ores, especially in the United States, in the latter half of the r 9th century and was superseded by the cyanide process and by smelting. Silver ore was ground to a fine paste with water in iron pans (fig. 2) by rotating iron shoes. The shoes were then raised a little, so as to agitate the pulp without further grinding, and mer cury was sprinkled into the pan. The agitation was continued until amalgamation was completed, and the end in view was assisted by heating the pan with steam and by the addition of chemicals, especially common salt and per sulphate. The amalgam was separated by diluting the pulp and stirring and finally by ning off the charge into large settling tanks. The tion pans were about sf t. in ameter and the charge was 2,000 or 3,000lb.

Complex silver ores containing minerals not amenable to amal gamation (such as the arsenical and antimonial sulphides, galena and blende) were roasted in furnaces at a red heat with common salt as a preliminary, when silver chloride was formed, a com pound from which the silver can be extracted by mercury.

Amalgamation of Gold Ores.

Thisis simpler and cheaper than that of silver ores. The machine used more than any other for the reduction of gold ores between 185o and 1925 was the stamp mill or battery (figs. 3 and 4). The stamp mill is worked on the principle of the pestle and mortar. The mortar is long and narrow, and ore, water and mercury are fed into it continuously.

Five heavy steel stampers, each weighing from Boo to 2,000lb., are ranged in a row in one mor tar. They are raised and let fall in succession, crushing the lumps of ore and driving the pulp through screens set in one side of the mortar. The old-fashioned wire screens were later replaced by perforated steel plates. The pulp coming from the screens is still further reduced by grinding in tube mills (rotating cylinders half filled with large pebbles, or lumps of unbroken ore) and then flows over sloping electro-silvered copper plates or tables, whose surface is amalgamated by rub bing with mercury. In older prac tice the copper plates were placed just outside the screens (fig. 4). The particles of gold adhere to the surface of the plate. Earlier still, amalgamated plates were fixed inside the mortar. The size and weight of stamps in the loth century caused excessive loss of mercury by flouring and the ad dition of mercury and the fixing of plates in the mortars were given up, the battery reverting to its original use as a mere crushing machine. Stamps forced down by springs or by power applied in various ways are sometimes used instead of gravity stamps and rotating crushers of various kinds (ball mills, tube mills, etc.) replace them in some new plants.

The clean-up takes place at intervals of a fortnight or a month.

The stamps are hung up, the sand is washed away, the amal gamated plates are scraped and the amalgam after careful cleaning is squeezed and retorted as usual (fig. 5). The percentage yield of gold varies with the conditions and the nature of the ore. Coarse gold is easily caught, but finely divided gold escapes in great part. Generally about 6o% of the gold in the ore was recovered by amalgamation, and the remainder by cyanide. Ores containing only 4oz. gold or even less per ton of ore have been treated at a profit.

On the Rand, amalgamation on copper plates had been generally discarded by 1925, owing to the difficulty of preventing the theft by Kafirs of the amalgam and the danger from mercury-poisoning. Instead of passing the crushed ore from the tube mills over amalgamated plates it is concentrated on a surface of corduroy, which retains the heavy particles including all coarse gold. This is a reversion to the primitive practice of the gold-diggers of Cal ifornia in 1849 who used their blankets in concentrating gold sands. At intervals the corduroy is taken up and washed in boxes and the coarse gold is extracted from the contents of the boxes by further concentration. The fine gold left in the ore is extracted in the cyanide plant. The corduroy also catches osmiridium and other valuable metals of the platinum group which had previously been lost. The changes on the Rand appear to foreshadow the end of the old amalgamation process. Amalgamation on copper plates lingers only in the Kolar gold-field in India, and in a few mills in Canada and some less important fields. (T. K. R.)

gold, silver, mercury, ores and ore