The assaying of ores may be performed either in the dry or moist way; the first is the most ancient, and, in many respects the most advantagageous, and consequently still continues to be mostly used.
Assays are made either in crucibles with the blast of the bellows, or in tests under a muffle.
The assay weights are always imaginary, sometimes an ounce represents a hundredweight on the large scale, and is subdivided into the same number of parts, as that hundredweight is in the great; so that the contents of the ore, obtained by the assay, shall accurately determine by such relative proportion the quantity to be expected from any weight of the ore on a larger scale.
_Roasting the Ore._
In the lotting of the ores care should be taken to have small portions from different specimens, which should be pulverized, and well mixed in an iron or brass mortar. The proper quantity of the ore is now taken, and if it contain either sulphur or arsenic it is put into a crucible or test, and exposed to a moderate degree of heat, till no vapor arises from it. To assist this volatilization some add a small quantity of powdered charcoal.
To assist the fusion of the ores, and to convert the extraneous matters connected with them into scoria, assayers use different kinds of fluxes. The most usual and efficacious materials for the composition of these are, borax, cream of tartar, nitre, sal ammoniac, common salt, glass, fluro-spar, charcoal powder, pitch, lime, litharge, etc., in different proportions.
As the whole process of which we are speaking is merely an experiment, made for the purpose of ascertaining what is the nature of the metal contained in the ore, and the proportion the former bears to the latter, the little additional expense incurred by employing animal instead of vegetable charcoal is not to be regarded, particularly when the increased fusibility of the ore, occasioned thereby is considered.
_Crude or White Flux._
This consists of 1 part of nitre and 2 of cream of tartar, well mixed together.
The above crude flux detonates by means of kindled charcoal, and if the detonation be effected in a mortar slightly covered it becomes black. It is a mixture of carbonate of potassa and charcoal.
_Cornish Reducing Flux._
Mix well together 10 oz. of cream of tartar, 3 oz. and 6 drs. of nitre, and 3 oz. and 1 dr. of borax.
_Cornish Refining Flux._
Deflagrate, and afterwards pulverize, 2 parts of nitre and 1 part of cream of tartar.
The above fluxes answer the purpose very well provided the ores be deprived of all their sulphur, or if they contain much earthy matters; because, in the latter case, they unite with them, and convert them into a thin glass, but if any quantity of sulphur remain, these fluxes unite with it, and form a liver of sulphur, which has the power of destroying a portion of all the metals; consequently the assay under such circumstances must be very inaccurate. The principal difficulty in assaying appears to be in the appropriation of the proper fluxes to each particular ore, and it likewise appears that such a discriminating knowledge can only be acquired from an extensive practice, or from a knowledge of the chemical affinities and actions of different bodies upon each other.
In assaying we are at liberty to use the most expensive materials to effect our purpose, hence the use of different saline fluxes; but in the working at large such expensive means cannot be applied, as by such processes the inferior metals would be too much enhanced in value, especially in working very poor ores. In consequence of which in smelting works, where the object is the production of metals in the great way, cheaper additions are used, such as limestone, feldspar, fluor-spar, quartz sand, slate, and slags. These are to be chosen according to the different views of the operator and the nature of the ores. Thus iron ores, on account of the argillaceous earth they contain, require calcareous additions, and the copper ores, rather slags or vitrescent stones, than calcareous earth.
_Humid Assay of Metallic Ores._
The mode of assaying ores for their particular metals by the dry way is deficient, so far as relates to pointing out the different substances connected with them, because they are always destroyed by the process for obtaining the assay metal. The essay by the moist way is more correct, because the different substances can be accurately ascertained.
_Dry Assay of Iron Ores._
Mix 100 grs. of the ore, thoroughly powdered, with from 30 to 100 grs. of calcined borax. The quality of the latter depends upon the quality of the ore, and is to be increased with the foreign matters. If the ore contains sulphur, it must first be roasted. The mixture is introduced into a crucible lined with charcoal, covered with powdered charcoal, on which is laid a piece of charcoal. The crucible is then closed, the cover luted on, and submitted to a white heat for an hour. The iron is found in the form of a button, and is not pure, but gives about the quality of the pig iron which will be obtained from the ore.
_Humid Assay of Iron Ore._
Fuch's method is accurate, and determines the relative quantity of protoxide and peroxide in an ore, which is often desirable. The only ores to which it cannot be applied are those containing arsenious acid, and this is not a very common ingredient.
Dissolve the ore in muriatic acid, and filter. Put into a small round-bottomed flask, and cork tightly until ready to boil it. Immerse a clean, weighed strip of copper, and, removing the cork, boil until the copper is no longer attacked. It is then taken out, washed, well wiped, and weighed. To ascertain the amount of peroxide multiply this weight by 40 and divide by 317. The quotient gives the amount sought.
To know the whole amount of iron in the ore, another portion is weighed out - say 1 gramme (about 15 grs.)--and dissolved as before; it is then digested with chlorate of potassa, by which all the iron is converted into peroxide, after which copper will decompose the whole. Multiply the loss by 28 and divide by 317; the quotient will give the whole amount of iron in the ore.
The presence of copper in the ore will make it appear poorer than it really is.
_Volumetric Assay of Iron Ore (Percy)._
Heat 10 grs. of iron-ore, finely pulverized, with strong hydrochloric acid, for 1/2 an hour, in a conicalshaped flask with a funnel inserted in the neck; when decomposition is complete dilute the solution with water acidulated with sulphuric acid, and add a few pieces of granulated zinc and boil until all traces of yellow color disappear, or the solution remains of a pale green tint, and free from fine particles of zinc. Transfer to a white porcelain dish, and dilute to 20 oz. with distilled water.
When cold it is ready for testing with the following solution.
Dissolve 50 grs. of crystallized permanganate of potassa in 20 oz. of distilled water, and keep it in a tightly corked bottle, marked "Standard Solution Permanganate of Potassa." To ascertain the standard of this solution, dissolve 10 grs. of iron piano wire in dilute hydrochloric acid in a narrow-mouth flask with gentle heat. Dilute to 10 oz. Take 1 oz. of the diluted iron solution and dilute to 20 oz. with distilled water in a white porcelain dish.
Allow the solution of permanganate to run slowly in from a graduated pipette, stirring constantly until the solution assumes a faint pink color. Record the amount used; this represents 1 gr. of metallic iron.
Proceed in the same manner to test the solution of ore first obtained, noting the amount required to produce the first tint of pink color. Divide this amount by the amounts required for 1 gr. of iron, and the result is the number of grains of metallic iron contained in the ore.
Mix a quintal of tin ore, previously washed, pulverized, and roasted till no arsenical vapor arises, with half a quintal of calcined borax, and the same quantity of pulverized pitch; these are to be put in a crucible lined with charcoal, which is placed in an air-furnace. After the pitch is burnt, give a violent heat for a quarter of an hour, and on withdrawing the crucible, the regulus will be found at the bottom. If the ore be not well washed from earthy matters, a larger quantity of borax will be requisite, with some powdered glass, and if the ore contain iron, some alkaline salt may be added.
_In the Humid Way._
Dissolve the ore in hot muriatic acid, pass through the solution a current of sulphuretted hydrogen in large excess. Allow the precipitate to subside; add to it, with the aid of heat, nitric acid until no sulphuretted hydrogen is given off. This transforms the tin into stannic acid; wash carefully, dry, and weigh. Stannic acid contains 78.61 per cent. of tin.
As most of the lead ores contain either sulphur or arsenic, they require to be well roasted. Take a quintal of roasted ore, with the same quantity of calcined borax, 1/2 a quintal of fine powdered glass, 1/4 of a quintal of pitch and as much clean iron filings. Line the crucible with wetted charcoal-dust, and put the mixture into the crucible, and place it before the bellows of a forge-fire. When it is red hot, raise the fire for 15 or 20 minutes, then withdraw the crucible, and break it when cold.
_In the Humid Way._
Powder the ore (Galena) finely. Moisten with fuming nitric acid and digest on the sand-bath. This converts the whole into sulphate of lead. Dilute with water and filter. The insoluble sulphate of lead will remain in the filter. Wash it thoroughly, dry it, and weigh--100 parts of sulphate of lead contain 73.56 parts of oxide of lead and 68.28% of metallic lead.
Take the assay weight of roasted ore, and mix it well with one-eighth part of charcoal-dust, put it into a strong luted earthen retort, to which must be fitted a receiver; place the retort in a furnace and raise the fire, and continue it in a violent heat for two hours; suffer it then to cool gradually, and the zinc will be found adhering to the neck of the retort in its metallic form.
_In the Humid Way (Percy)._
Take 20 grs. of the ore (finely pulverized) to be assayed. Digest it for 1 hour in nitric acid 1 part, water 2 parts, with a few drops of hydrochloric acid; add carbonate of ammonia dissolved in liquid ammonia until the reaction is strongly alkaline. Digest for half an hour, dilute with an equal bulk of distilled water; filter and mark the filtrate Sol. A.
Make a standard solution of zinc by dissolving 10 grs. of pure zinc in nitric acid and diluting to 10 oz. Sol. B.
Make a solution of sulphide of sodium, 1 oz. of saturated solution to 10 oz. of distilled water. Sol. C.
Take of solution chloride of iron, 1/2 oz.; distilled water, 5 oz.: add aqua ammonia, separate all of the iron. Shake. Sol. D.
Take of solution B. 1 oz.; dilute to 3 oz.; add of solutions D, 1 oz.; take in a graduated pipette of solution C, and add gradually to the mixture of B and D (stirring rapidly all the while), until the flocculent iron begins to change color to grayish black. Make a memorandum of the number of graduations of solution C required. This is the amount of sulphide of sodium necessary to precipitate 1 gr. of metallic zinc.
Take 1/2 of solution A (diluted to 12 oz.) equal 6 oz.; add of solution D, 2 oz.; then with graduated pipette run in slowly the solution C until the flocculent iron begins to change color as before. The number of graduations required, divided by the number used in the former experiment, indicate the number of grains of metallic zinc in 10 grs. of the ore, and represent the per centage likewise.
Take an exact troyounce of the ore, previously pulverized, and calcine it well; stir it all the time with an iron rod without removing it from the crucible; after the calcination add an equal quantity of borax, 1/2 the quantity of fusible glass, 1/4 the quantity of pitch, and a little charcoal-dust; rub the inner surface of the crucible with a paste composed of charcoal- dust, a little fine powdered clay, and water. Cover the mass with common salt, and put a lid upon the crucible, which is to be placed in a furnace; the fire is to be raised gradually till it burns briskly, and the crucible continued in it for 1/2 hour, stirring the metal frequently with an iron rod; and when the scoria which adheres to the rod appears clear, then the crucible must be taken out and suffered to cool, after which it must be broken and the regulus separated and weighed. This is called black copper; to refine which equal parts of common salt and nitre are to be well mixed together. The black copper is brought into fusion, and a teaspoonful of the flux is thrown upon it which is repeated 3 or 4 times, when the metal is poured into an ingot mould and the button is found to be fine copper.
_In the Humid Way._
Make a solution of vitreous copper ore in 5 times its weight of concentrated sulphuric acid and boil it to dryness; add as much water as will dissolve the vitriol thus formed. To this solution add a clean bar of iron, which will precipitate the whole of the copper in its metallic form. If the solution be contaminated with iron, the copper must be redissolved in the same manner and precipitated again. The sulphur may be separated by filtration.
_Volumetric Assay of Copper Ores._
Dissolve 10 grs. of the copper ore finely pulverized and moistened with strong sulphuric acid, in strong nitric acid, adding the acid gradually; and when the fumes of hyponitric acid cease to be evolved, add a small amount of water and boil for a few minutes. Dilute to 10 oz. and treat with ammonia in excess, and it will become of a deep blue color. Set aside to cool, and prepare the following solution: Dissolve 500 grs. of granulated cyanide of potassium in 20 oz. of distilled water, and keep in a tight-stoppered bottle in the dark. Mark "Standard Solution Cyanide of Potassium". To ascertain the standard of this solution, dissolve 10 grs. of electrotype copper in dilute nitric acid and boil to expel hyponitric acid fumes, and dilute to 10 oz. with distilled water. Take of this solution 1 oz. and dilute to 5 oz. with distilled water, and allow the standard cyanide solution to flow very slowly into it at intervals, from a graduated pipette, and note the amount used to render it nearly colorless. This process takes from 1/2 to 3/4 of an hour. Proceed in the same manner to test the solution of ore first obtained, noting the amount required to reduce the color to a faint lilac. Divide this amount by the amount found required for 1 gr. of metallic copper, and the result is the number of grains of metallic copper in the ore tested.
If the ore be mineralized by sulphur, or sulphur and iron, a previous roasting will be necessary. The strong ores require no roasting, but only to be reduce to a fine powder. Take the assay weight and mix it with half the quantity of calcined borax, and the same of pounded glass; line the crucible with charcoal, melt it as quickly as possible, and when well done, take out the crucible and let it cool gradually. The regulus will be found at the bottom.
_In the Humid Way._
Bismuth is easily soluble in nitric acid or aqua regia. Its solution is colorless and is precipitable by the addition of pure water; 118 grs. of the precipitate from nitric acid, well washed and dried, are equal to 100 of bismuth in its metallic form.
Take a common crucible, bore a number of small holes in the bottom, and place it in another crucible a size smaller, luting them well together; then put the proper quantity of ore in small lumps into the upper crucible, and lute thereon a cover; place these vessels on a hearth and surround them with stones about 6 in. distant from them, the intermediate space must be filled with ashes, so that the undermost crucible may be covered with them; but upon the upper charcoal must be laid and the whole made red hot by the assistance of hand bellows. The antimony being of easy fusion is separated, and runs through the holes of the upper vessel into the inferior one, where it is collected.
_Humid Assay of Arseniated Antimony._
Dissolve the ore in aqua regia; the sulphur is seperated by filtration. Evaporate the solution to dryness and heat below redness until all the nitric acid is expelled. The resulting antimonic acid contains 76.33 per cent. of metallic antimony.
The regulus is obtained by mixing the calx or ore of manganese with oil, making it into a ball, and putting it into a crucible lined with powdered charcoal 1 10th of an inch on the sides, and 1/4 of an inch at bottom; then filling the empty space with charcoal- dust, covering the crucible with another inverted and luted on, and exposing it to the strongest heat of a forge for an hour or more. The ore is very difficult to reduce.
This assay is made by sublimation in close vessels. Beat the ore into small pieces and put them into a matrass, which place in a sand-pot with a proper degree of heat. The arsenic sublimes in this operation and adheres to the upper part of the vessel; when it must be carefully collected, with a view to ascertain its weight. A single sublimation will not be sufficient. It is better to perform the first sublimation with a moderate heat, and afterwards bruise the remainder and expose it to a stronger heat. The addition of charcoal is useful.
_In the Humid Way._
Digest the ore in muriatic acid, adding nitric by degrees, to help the solution. The sulphur will be found on the filter; the arsenic will remain in the solution, and may be precipitated in its metallic form by boiling with a strip of copper.
The ores must be well roasted to expel the sulphur and arsenic; the greener the calx proves during this torrefaction, the more it abounds in the nickel; but the redder it is, the more iron it contains. The proper quantity of this roasted ore is fused in an open crucible, with twice or thrice its weight of black flux, and the whole covered with common salt. By exposing the crucible to the strongest heat of a forge fire, and making the fusion complete, a regulus will be produced. This regulus is not pure, but contains a portion of arsenic, cobalt, and iron. Of the first it may be deprived by a fresh calcination, with the addition of powdered charcoal; and of the second by scorification. But it is with difficulty that it is entirely freed from the iron.
_In the Humid Way._
By solution in nitric acid it is freed from its sulphur; and by adding water to the solution, bismuth, if any, may be precipitated; as may silver, if contained it, by muriatic acid; and copper, when any, by iron.
To separate cobalt from nickel, the two oxides are dissolved in muriatic acid; the solution diluted with distilled water. The liquor is saturated with chlorine, and when gold, an excess of precipitated carbonate of baryta added. It is then set aside for 18 hours, when the cobalt will be precipitated as sesquioxide, while the nickel will remain in solution.
Free them as much as possible from earthy matters by well washing, and from sulphur and arsenic by roasting. The ore thus prepared is to be mixed with 3 parts of black flux, and a little decrepitated sea-salt; put the mixture in a lined crucible, cover it and place it in a forge-fire, or in a hot furnace, for this ore is very difficult of fusion.
When well fused, a metallic regulus will be found at the bottom, covered with a scoria of a deep blue color; as almost all cobalt ores contain bismuth, this is reduced by the same operation as the regulus of cobalt; but as they are incapable of chemically uniting together, they are always found distinct from each other in the crucible. The regulus of bismuth, having a greater specific gravity, is always at the bottom, and may be separated by a blow with a hammer.
_In the Humid Way._
Make a solution of the ore in nitric acid, or aqua regia, and evaporate to dryness; the residuum, treated with the acetic acid, will yield to it the cobalt; the arsenic should be first precipitated by the addition of water.
The calciform ores of mercury are easily reduced without any addition. A quintal of the ore is put into a retort, and a receiver luted on, containing some water; the retort is placed in a sand-bath, and a sufficient degree of heat given it, to force over the mercury which is condensed in the water of the receiver.
_Sulphuretted Mercurial Ores._
The sulphurous ores are assayed by distillation in the manner above, only these ores require an equal weight of clean iron-filings to be mixed with them, to disengage the sulphur, while the heat volatilizes the mercury, and forces it into the receiver. These ores should likewise be tried for cinnibar, to know whether it will answer the purpose of extracting it from them; for this a determinate quantity of the ore is finely powdered and put into a glass vessel, which is exposed to a gentle heat at first, and gradually increased till nothing more is sublimed. By the quantity thus acquired, a judgment may be formed whether the process will answer. Sometimes this cinnabar is not of so lively a color as that which is used in trade; in this case it may be refined by a second sublimation, and if it be still of too dark a color, it may be brightened by the addition of a quantity of mercury, and subliming it again.
_Humid Assay of Cinnabar._
The stony matrix should be dissolved in nitric acid, and the cinnabar being disengaged, should be boiled in 8 or 10 times its weight of aqua regia, composed of 3 parts of nitric, and 1 of muriatic acid. The mercury may be precipitated in the metallic form by zinc.
Take the assay quantity of the ore finely powdered, and roast it well in a proper degree of heat, frequently stirring it with an iron rod; then add to it about double the quantity of granulated lead, put it in a covered crucible, and place it in a furnace; raise the fire gently at first, and continue to increase it gradually, till the metal begins to work; if it should appear too thick, make it thinner by the addition of a little more lead; if the metal should boil too rapidly, the fire should be diminished. The surface will be covered by degrees with a mass of scoria, at which time the metal should be carefully stirred with an iron hook heated, especially towards the border, lest any of the ore should remain undissolved; and if what is adherent to the hook when raised from the crucible melts quickly again, and the extremity of the hook, after it is grown cold is covered with a thin, shining smooth crust, the scorification is perfect; but, on the contrary, if, while stirring it, any considerable clamminess is perceived in the scoria, and when it adheres to the hook, though red hot, and appears unequally tinged and seems dusty or rough, with grains interspersed here and there, the scorification is incomplete; in consequence of which the fire should be increased a little and what adheres to the hook should be gently beaten off, and returned with a small ladle into the crucible again. When the scorification is perfect, the metal should be poured into a cone, previously rubbed with a little tallow, and when it becomes cold, the scoria may be separated by a few strokes of a hammer. The button is the produce of the assay.
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