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Assaying (Geous and consequently still)

geous, 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.

Assay Weights.

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.

Fluxes.

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, fluor-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. 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 cop per 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 assay 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 This consists of 1 part of nitre and 2 of cream arsenious acid, and this is not a very common inof tartar, well mixed together. Black Flux.

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 gredient.

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 31.7. 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 31.7; 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 an hour, in a conical-shaped 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 follow- the zinc will be found adhering to the neck of the ing 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.

Tin Ores.

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.

Lead Ores.

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, a quintal of fine powdered glass, 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 68.285 of metallic lead.

Zinc Ores.

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 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, 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 solution 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 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.

Copper Ores.

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, the quantity of fusible glass, 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 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 re-dissolved in the same manner and precipitated again. The sulphur may be separated by filtration.