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ASSAYING . To " assay " (or " See also:essay"; Fr. essayer) is in See also:general to try, or See also:attempt, so to make trial or test. In a restricted sense the See also:term assaying is applied in metallurgy to the determination of the amount of See also:gold or See also:silver in ores or See also:alloys; in this See also:article, however, it will be used in a wider technical signification, and will include a description of the methods for the quantitative determination of those elements in ores which affect their value in metallurgical operations. It would be impossible to give in detail here all the precautions necessary for the successful use of the methods, and the descriptions will therefore be confined to the principles involved and the general manner in which they are applied to secure the desired results. Gold and Silver.—Ores containing gold or silver are almost invariably assayed in the dry way; that is, by See also:fusion with appropriate fluxes and ultimate separation of the elements in the metallic See also:form. One of the customs which has grown out of our See also:peculiar See also:system of weights is the form of statement of the results of such an assay. Instead of expressing the amounts of gold and silver in percentages of the See also:weight of ore, they are expressed in ounces to the ton, the See also:ounce being the See also:troy ounce and the ton that of 2000 See also:avoirdupois pounds. To simplify calculation and to enable the assayer to use the metric system of weights employed in all chemical calculations, the " assay ton " (" A.T. " = 29.166 grammes) has been devised, which bears the same relation to the ton of 2000 lb avoirdupois that one milligram does to the troy ounce; when one assay ton of ore is used, each milligram of gold or silver found represents one ounce to the ton. The assay of an ore for gold or silver consists of two operations. In the first the gold or silver is made to combine or alloy with metallic See also:lead, the other constituents of the ore being separated from the lead as slag. In the second, the lead See also:button containing the gold or silver is cupelled and the resulting gold or silver button is weighed. The first is conducted in one of two ways, known respectively as the crucible method and the scorification method. The crucible method is generally used for ores containing gold in small amounts and for certain classes of silver ores. The amount of ore taken for assay is generally one-See also:half " A.T.," but in very See also:low-grade ores one, two, and sometimes even four "A.T.s" are used. In the scorification method one-tenth of an " A.T." is the amount commonly taken. While in both methods the same result is sought, the means employed are quite different. In the scarification method the ore is mixed in the scorifier (a shallowdish of burned See also:clay) with from ten to twenty times its weight of granulated metallic lead (test lead) and a little See also:borax See also:glass, and heated in a muffle, the front of which is at first closed. When the lead melts and begins to oxidize, the lead See also:oxide, or so-called litharge, combines with or dissolves the non-metallic and readily oxidizable constituents of the ore, while the gold and silver alloy with the lead. As the slag thus formed flows off to the sides of the scorifier, the assay clears and the melted metallic lead forms an " See also:eye in the See also:middle. The See also:door of the muffle is then opened and the current of See also:air which is See also:drawn over the scorifier rapidly oxidizes the lead, while the melted litharge gradually closes over the See also:metal. When the " eye " has quite disappeared the door is closed and the temperature raised to make the slag very liquid. The scorifier is taken from the muffle in a pair of See also:tongs and the contents poured into a See also:mould, the lead forming a button in the bottom while the slag floats on See also:top. When' See also:cold, the contents of the mould are taken out and the lead button hammered into the form of a See also:cube, the slag, which is glassy and brittle, separating readily from the metal, which is then ready for cupellation. In the crucible method the ore is mixed with from once to twice its weight of See also:flux, which varies in See also:composition, but of which the following may be taken as a type: See also:Sodium bicarbonate See also:Potassium carbonate. Powdered borax See also:Flour . . Litharge The mixture is charged into a See also:round clay crucible from too mitt to 125 mm. high, and heated either in a muffle or in a crucible See also:furnace at a gradually increasing See also:heat for See also:forty or fifty `minutes: At the expiration of this See also:time, when the See also:charge should be perfectly liquid and in a tranquil See also:state of fusion, the crucible is removed from the furnace and the contents are poured into a mould. The resulting lead button hammered into shape and carefully cleansed from slag is ready for the cupel. If the button is too large for cupellation, or if it is hard, it may be scorified either alone or mixed with test lead before cupellation. The See also:character and amount of the flux necessarily depend upon the character of the ore, the See also:object being to concentrate in the lead button all the gold and silver while dissolving and carrying off in the slag the other constituents of the ore. Under the most favourable conditions there is a slight loss of gold and silver in the fusion, the scorification and the cupellation, both by absorption in the slag and by actual volatilization and absorption in the cupel. In ores containing. much See also:copper, this metal is largely concentrated in the lead button, making it hard, and necessitating repeated scorifications and, in some cases, a preliminary removal of the copper by See also:solution of the ore in nitric See also:acid. This leaves the gold in the insoluble See also:residue, which is filtered off, and the silver in the solution is thrown down by hydrochloric acid. The resulting precipitate of silver chloride is filtered, and the residue and the precipitate are scorified together. Ores containing much See also:arsenic+ or See also:sulphur are generally roasted at a low heat and the assay is made on the roasted material. The See also:process of cupellation is briefly as follows:—The gold alloy is fused with a quantity of lead, and a little silver if silver is already See also:present. The resulting alloy, which is called the lead button, is then submitted to fusion on a very porous support, made of See also:bone-ash, and called a cupel. The fusion being effected in a current of air, the lead oxidizes. The heat is sufficient to keep the resulting lead oxide fused, and the porous cupel has'the See also:property ofabsorbing melted lead oxide without taking up any of the metallic globule, exactly in the same way that blotting-See also:paper will absorb See also:water whilst it will not See also:touch a globule of See also:mercury. The heat being continued, and the current of air always passing over the See also:surface of the melted lead button, and the lead oxide being sucked up by the cupel as fast as it is formed, the metallic globule rapidly diminishes in See also:size until at last all the lead has been got rid of. Now, if this were the only See also:action, little See also:good would have been gained, for we should simply have put lead into the gold alloy, and then taken it out again;'but another action goes on whilst the lead is oxidizing in the current . 8 parts.,. of air. Other metals, except the silver and gold, also oxidize, and are carried by the melted litharge into the cupel. If the lead is therefore rightly proportioned to the See also:standard of alloy, the resulting button will consist of only gold and silver, and these are separated by the operation of parting, which consists in boiling the alloy (after See also:rolling it to a thin See also:plate) in strong nitric acid, which dissolves the silver and leaves the gold as a coherent sponge. To effect this parting properly, the proportion of silver to gold should be as 3 to I. The operation by which the alloy is brought to this standard is termed quartation or inquartation, and consists in fusing the alloy in a cupel with lead and the quantity of See also:fine silver or fine gold necessary to bring it to the desired composition. Lead.— The " dry " or See also:fire assay for lead is largely used for the valuation of lead ores, although it is being gradually replaced by volumetric methods. One See also:part of the ore is mixed with from three to five parts of a flux of the following composition: Potassium carbonate . . . 40.6 % Sodium bicarbonate . . 31.3 „ Borax . 15.6 „ Flour . . 12.5 „ The mixture is charged into a clay crucible and heated for twenty minutes at a good red heat. When the mixture has been in a tranquil state of fusion for a few minutes it is poured into a mould. When cold, the button is hammered, cleaned carefully from slag, and weighed. The proportion is calculated from the amount of ore used, and the result is expressed in parts in a See also:hundred or percentage of the ore. Various impurities, such as copper, See also:antimony and sulphur, go into the lead button, so that the result is generally too high. The most accurate method for the determination of lead in ores is the See also:gravimetric method, in which it is weighed as lead sulphate after the various impurities have been separated. Nearly all lead ores contain more or less sulphur; and as in the process of solution in nitric acid this is oxidized to sulphuric acid which unites with the lead to form the very insoluble lead sulphate, it is simpler to add sulphuric acid to convert all the lead into sulphate and then evaporate until the nitric acid is expelled. The salts of See also:iron, copper, &c., are then dissolved in water and filtered from the insoluble See also:silica, lead sulphate, and See also:calcium sulphate, which are washed with dilute sulphuric acid. The insoluble See also:matter is treated with a hot solution of alkaline ammonium acetate, which dissolves the lead sulphate, the other materials being separated by filtration. The lead sulphate, re-precipitated in the filtrate by an excess of sulphuric acid and See also:alcohol, is then filtered on an See also:asbestos See also:felt in a See also:Gooch crucible, washed with dilute sulphuric acid and alcohol, ignited, and weighed. Lead sulphate contains 68.3o % of metallic lead.
There are several volumetric methods for assaying lead ores, but the best known is that based on the precipitation of lead by ammonium molybdate in an acetic acid solution. The lead sulphate, obtained as described above and dissolved in ammonium acetate, is acidulated with acetic acid diluted with hot water and heated to boiling-point. A standardized solution of ammonium molybdate is then added from a burette. As See also:long as the solution contains lead, the addition of the molybdate solution causes a precipitation of See also: 
It is standardized by dissolving a weighed amount of lead sulphate in ammonium acetate and proceeding as described above.
See also:Zinc.—Chemically the ores of zinc consist of the silicates, See also:carbonates. oxides. and sulphides of zinc associated with othermetals, some of which complicate the methods of assay. The most See also:modern and the most generally accepted method is volumetric, and is based on the reaction between zinc chloride and potassium ferrocyanide, by which insoluble zinc ferrocyanide and soluble potassium chloride are formed; the presence of the slightest excess of potassium ferrocyanide is shown by a brownish tint being imparted by the solution to a drop of See also:uranium nitrate. The ore (o.5 gramme) is digested with a mixture of potassium nitrate and nitric acid. A saturated solution of potassium chlorate in strong nitric acid is added, and the See also:mass evaporated to dryness. It is then heated with a mixture of ammonium chloride and See also:ammonia, filtered and washed with a hot dilute solution of the same mixture. The filtrate diluted to 200 cc. is carefully neutralized with hydrochloric acid, and excess of 6 cc. of the strong acid is added, and the solution saturated with See also:hydrogen sulphide, which precipitates the copper and See also:cadmium, metals which would otherwise interfere. Without filtering, the standard solution is added from a burette, and from time to time a drop of the solution is removed on the glass stirring-rod and added to a drop or two of a strong solution of uranium nitrate, previously placed on a white porcelain plate. The appearance of a See also: Copper.— The fire assay for copper ores was abandoned years ago and the electrolytic method took its See also:place; this in turn is now largely replaced by volumetric methods. In the electrolytic method from o.5 to 5 grammes of ore are treated in a See also:flask or See also:beaker, with a mixture of ro cc. of nitric and ro cc. of sulphuric acid, until thoroughly decomposed. . When this liquid is cold it is diluted with cold water, heated until all the soluble salts are dissolved, transferred to a tall, narrow beaker, and diluted to about 150 cc. The electrodes are attached to a See also:frame connected with the See also:battery and the beaker is placed on a See also:stool, which can be raised so that the electrodes are immersed in the liquid and reach the bottom of the beaker. The electrodes consist of two cylinders of See also:platinum (placed one inside the other) about 75 mm. high, the smaller of the two 37. mm. and the larger 50 mm. in See also:diameter, both pierced with to to 12 holes 5 mm. in diameter, evenly distributed over the surfaces to facilitate See also:diffusion of the liquids. The surfaces of the cylinders are roughened with a See also:sand blast to increase the areas and make the deposited metals adhere more firmly. Each See also:cylinder has a platinum See also:wire fused to the upper circumference to connect with a clamp from which a wire leads to the proper See also:pole of the battery. The smaller cylinder is generally the negative electrode on which the copper is deposited. The framework carrying the clamps is arranged so that a number of determinations may be made at one time, the wires from the clamps See also:running from a rheostat, so arranged that currents of any strength may be used simultaneously. The cylinder, having been carefully weighed, is placed in position, the beaker containing the solution is adjusted, and the current passed until all the copper is precipitated. This generally requires from two to twelve See also:hours. The cylinders are then removed from the solution and washed with distilled water, the one holding the deposited copper being washed with alcohol, dried and weighed; the increase in weight represents the copper contents of the ore. The deposited copper should be firmly adherent and See also:bright rosy red in colour. Silver, arsenic and cadmium, if present, are precipitated with the copper and affect the accuracy of the results; they should be removed by See also:special methods. Volumetric methods are more expeditious and require less apparatus. The potassium See also:cyanide method is based on the fact that, when potassium cyanide is added to an ammoniacal solution of a See also:salt of copper, the insoluble copper cyanide is formed, the end of the reaction being indicated by the disappearance of the See also:blue colour of the solution. One gramme of the ore is treated in a flask with a mixture of nitric and sulphuric acids and evaporated until all the nitric acid is expelled. After cooling a little, water is added, and then a few grammes of See also:aluminium See also:foil free from copper. On this foil the copper in the solution is all precipitated by electrolytic action in a few minutes, and the aluminium is dissolved by the addition of an excess of sulphuric acid. Water is added, and as soon as the See also:gangue and copper particles have settled the clear solution is decanted, and the residue washed several times in the same way. The copper is then dissolved in 5 ce. of nitric acid; if silver is present a drop or two of hydrochloric acid is added, the solution diluted to about 50 cc., and filtered. To the filtrate (or, if no silver is present, to the diluted nitric acid solution) ro cc. of ammonia are added, and a standard solution of potassium cyanide is run in from a burette until the blue colour has nearly disappeared. The solution is filtered to get rid of the precipitate, and the titration is finished in the nearly clear filtrate, which should be always about 200 cc. in See also:volume. The titration is See also:complete when the blue colour is so faint that it is almost imperceptible after the flask has been vigorously shaken. The potassium cyanide solution is standardized by dissolving o• 5 gramme of pure copper in 5 cc. of nitric acid, diluting, adding 10 cc. of ammonia, and titrating exactly as described above. When potassium iodide is added to a solution of cupric acetate, the reaction (Cu(C2H302)2+ 2KI= CuI + 2K(C2H302) + I takes place; that is, for each See also:atom . of copper one atom of See also:iodine is liberated. If a solution of sodium thiosulphate (hyposulphite) is added to this solution, hydriodic acid, sodium iodide and tetrathionate are formed; and if a little See also:starch solution has been added, the end of the reaction is indicated by the disappearance of the blue colour, due to the iodide of starch. The amount of iodine liberated is therefore a measure of the copper in the solution, and when the sodium thiosulphate has been carefully standardized the method is extremely accurate. The ore is treated as described in the cyanide method until the copper precipitated by the aluminium foil has been washed and dissolved in 5 cc. of nitric acid; then o•25 gramme of potassium chlorate is added, and the solution boiled nearly dry to oxidize any arsenic present to arsenic acid. The solution is cooled, 50 cc. water added, then 5 cc. ammonia, and the solution is boiled for five minutes. Next 5 cc. of glacial acetic acid are added, the solution cooled, and 5 cc. of a solution of potassium iodide (300 grammes to the litre) and the standard solution of sodium thiosulphate run in from a burette until the brown colour has nearly disappeared. A few drops of starch solution are then added, and when the blue colour has nearly vanished a drop or two of methyl See also:orange makes the end reaction very See also:sharp. The thiosulphate solution is standardized by dissolving o•3 to 0.5 gramme of pure copper in 3 cc. of nitric acid, adding 50 cc. of water ,and 5 cc. of ammonia, and titrating as above after the addition of 5 cc. of glacial acetic acid and 5 ce. of the potassium iodide solution. Iron.—The methods used in the assay for iron are volumetric, and are all based on the property possessed by certain reagents of oxidizing iron from the ferrous to the ferric state. Two salts are in See also:common use for this purpose, potassium permanganate and potassium bichromate. It is necessary in the first place, after the ore is in solution, to reduce all the iron to the ferrous See also:condition; then the carefully standardized solution of the oxidizing reagent is added until all the iron is in the ferric state, the volume of the standard solution used being the measure of the iron contained in the ore. The end of the reaction when potassium permang nate is employed is known by the See also:change in colour of the solution. As the solution of potassium permanganate, which is deep red in colour, is dropped into the colourless iron solution, it is quickly decolorized while the iron solution gradually assumes a yellowish tinge, the first drop of the permanganate solution in excess giving it a See also:pink tint. With potassium bichromate solution, which is yellow, the iron solution becomes See also:green from the See also:chromium chloride or sulphate formed, and the See also:ASSELIJN end of the reaction is determined by removing a drop of the solution on the stirring-rod and' adding it to a drop of a dilute solution of potassium ferricyanide on a white See also:tile. So long as the solution contains a ferrous salt, the drop on the tile changes to blue; hence the See also:absence of a blue coloration indicates the complete oxidation of all the ferrous salt and the end of the reaction. One gramme of ore is usually taken for assay and treated in a small flask or beaker with 10 cc. of hydrochloric acid. All the iron in the ore generally dissolves upon See also:heating, and a white residue is See also:left. Occasionally this residue contains a small amount of iron in a difficultly soluble form; in that See also:case the solution is slightly diluted with water and filtered into a larger flask. The residue in the See also:filter is ignited and fused with a little sodium carbonate and nitrate, or with sodium peroxide. The product is treated with water, filtered, and the residue dissolved in hydrochloric acid and added to the See also:main solution. This solution, which should not exceed 50 cc. or 75 cc. in volume, contains the iron in the ferric state and is ready for reduction. In the reduction by metallic zinc; about 3 grammes of granulated or foliated zinc are placed in the flask, which is closed with a small See also:funnel; when the iron is reduced, add 10 cc. of sulphuric acid, and as soon as all the zinc is dissolved the solution is ready for titration. In the reduction by stannous chloride the solution of the ore in the flask is heated to boiling, and a strong solution of stannous chloride is added until the solution is completely decolorized; then 6o cc. of a solution of mercuric chloride (5o grammes to the litre) are run in and the contents of the flask poured into a dish containing boo cc. of water and 6o cc. of a solution containing 200 grammes of manganous sulphate, 1 litre of phosphoric acid (1.3 sp. gr.), 400 cc. of sulphuric acid, and rboo cc. of water. The solution is then ready for titration with the standard permanganate solution. The permanganate or bichromate solution is standardized by dissolving 0.5 of a gramme of pure iron wire in a flask, in hydrochloric acid, oxidizing it with a little potassium chlorate; boiling off all traces of See also:chlorine, deoxidizing by one of the methods described above, and titrating with the solution. As the wire always contains impurities, the See also:absolute amount of iron in the wire must be determined and the correction made accordingly. Pure oxalic acid may also be used, which, in the presence of sulphuric acid, is oxidized by the standard solution according to the reaction:— . 5(H2C2O42H20) +3H2SO4+2 KM nO4 =10002-f-2MnSO4 +K2SO4+18H2O The reaction in case of ferrous sulphate is e 1OFeSO4+2KM nO4+8 H2SO4 = 5Fe2(SO4) 3+K2SOa +214 nSO4 +8H20 ; that is, the same amount of potassium permanganate is required to oxidize 5 molecules of oxalic acid that is necessary to oxidize 10 molecules of iron in the form of ferrous sulphate to ferric sulphate, or 63 parts by weight of oxalic acid equal 56 parts by weight of metallic iron. Ammonium ferrous sulphate may also be used: it contains one-seventh of its weight of iron. ,(A. A. Additional information and CommentsThere are no comments yet for this article.
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