A Text-book of Assaying: For the Use of Those Connected with Mines Part 23

The _standard solutions of acid and alkali_ used for this determination may be one-fifth normal. 200 c.c. of the normal solution should be diluted to 1 litre in each case, 1 c.c. of the resulting solutions would be equivalent to 8 milligrams of soda (NaHO) or 5.6 milligrams of lime, CaO. It must be remembered this refers to the pure bases in each case.

Suppose it is desired to report as so many lbs. of lime to the short ton (2000 lbs.) of ore. Since 1 c.c. of the standard solution is equivalent to 5.6 milligrams of lime, if we take 2000 times this weight of ore (_i.e._ 11,200 milligrams or 11.2 grams) for the a.s.say, each c.c. of standard solution will be equivalent to 1 lb. of lime to the short ton.[45]

~Total acidity.~--Weigh out 11.2 grams of the ore, place them in a four-inch evaporating dish and measure on to it from a burette 10 or 20 c.c. of the standard solution of soda. Stir the soda solution into the ore and allow to stand for 15 or 20 minutes with occasional stirring.

Stir up with 30 or 40 c.c. of water, float a piece of litmus paper on the liquid and t.i.trate with the standard solution of acid. If the ore is strictly neutral the quant.i.ty of "acid" required to redden the litmus will be the same as the quant.i.ty of "soda" originally used. If the ore is acid, less acid will be used. For example, if 10 c.c. of soda were used and only 7 c.c. of acid were required, the ore will have done the work of the remaining 3 c.c. of acid. And the ton of ore will require 3 lbs. of lime to neutralise its acidity.

~Acidity after was.h.i.+ng.~--Take 11.2 grams of the ore; wash thoroughly with water and immediately treat the residue, without drying, exactly as just described.

~Examination of cyanide solutions for metals, &c.~--Take a measured quant.i.ty of the solution, say 20 c.c.[46] and evaporate in a small dish with, say, half a c.c. of strong sulphuric acid. Evaporate at first, on a water-bath in a well ventilated place, but finish off with a naked Bunsen flame, using a high temperature at the end in order to completely decompose the more refractory double cyanides. Allow to cool; moisten with strong hydrochloric acid; warm with a little water and test for the metals in the solution by the ordinary methods. Since the quant.i.ties of the metals likely to be present may be given in milligrams the work must be carefully performed. It may be worth while to determine the proportions of lime and magnesia as well as those of the metals proper.

Or the 20 c.c. of cyanide liquor may be evaporated with 5 c.c. of strong nitric acid to dryness and gently ignited and the residue taken up with 2 or 3 c.c. of strong hydrochloric acid.

Copper, iron, and zinc can be rapidly determined in such a solution, as follows. Dilute with water to 10 or 15 c.c., add an excess of ammonia, and filter. The precipitate will contain the iron as ferric hydrate; dissolve it in a little hot dilute sulphuric acid: reduce with sulphuretted hydrogen; boil off the excess of gas, cool and t.i.trate with standard pota.s.sium permanganate (p. 236). Determine the copper in the filtrate colorimetrically (p. 203); but avoid further dilution. Then add dilute hydrochloric acid, so as to have an excess of 4 or 5 c.c. after neutralising the ammonia; add some clean strips of lead foil, and boil until the solution has for some time become colourless. t.i.trate with standard pota.s.sium ferrocyanide (p. 263) without further dilution, and bearing in mind that at most only one or two c.c. will be required.

~Examination of an ore for "cyanicides."~--Place 100 grams of the ore with 200 c.c. of a cyanide solution of known strength (say .1 or .2 per cent.) in a bottle and agitate for a definite time, such as one or two days. Filter off some of the liquor and a.s.say for cyanide, using say 20 c.c. Calculate how much cyanide has been destroyed in the operation.

Evaporate 20 c.c. with sulphuric or nitric acid and examine for metal.

Test another portion for sulphides, &c.

The student who has mastered the methods of a.s.saying can greatly improve himself by working out such problems as the above.

PLATINUM.

Platinum occurs in nature in alluvial deposits a.s.sociated with gold and some rare metals, generally in fine metallic grains, and, occasionally, in nuggets. It is a grey metal with a high specific gravity, 21.5 when pure and about 18.0 in native specimens. It is fusible only at the highest temperature, and is not acted on by acids.

It is dissolved by warm aqua regia, forming a solution of "platinic chloride," H_{2}PtCl_{6}. This substance on evaporation remains as a brownish red deliquescent ma.s.s; on drying at 300 C. it is converted into platinous chloride, PtCl_{2}, and becomes insoluble, and at a higher temperature it is converted into platinum. All platinum compounds yield the metal in this way. Platinic chloride combines with other chlorides to form double salts, of which the ammonic and pota.s.sic platino-chlorides are the most important.

Platinum alone is not soluble in nitric acid; but when alloyed with other metals which dissolve in this acid it too is dissolved; so that in gold parting, for example, if platinum was present, some, or perhaps the whole of it would go into solution with the silver. Such alloys, however, when treated with hot sulphuric acid leave the platinum in the residue with the gold.

Platinum is detected when in the metallic state by its physical characters and insolubility in acids. In alloys it may be found by dissolving them in nitric acid or in aqua regia, evaporating with hydrochloric acid, and treating the filtrate with ammonic chloride and alcohol. A heavy yellow precipitate marks its presence.

The a.s.say of bullion, or of an alloy containing platinum, may be made as follows: Take 0.2 gram of the alloy and an equal weight of fine silver, cupel with sheet lead, and weigh. The loss in weight, after deducting that of the silver added, gives the weight of the base metals, copper, lead, &c. Flatten the b.u.t.ton and part by boiling with strong sulphuric acid for several minutes. _When cold_, wash, anneal, and weigh. The weight is that of the platinum and gold. The silver may be got by difference. Re-cupel the metal thus got with 12 or 15 times its weight of silver, flatten and part the gold with nitric acid in the usual way (see under _Gold_), and the platinum will dissolve. The gold may contain an alloy of osmium and iridium; if so, it should be weighed and treated with aqua regia. The osmiridium will remain as an insoluble residue, which can be separated and weighed. Its weight deducted from that previously ascertained will give the weight of the gold.

When the platinum only is required, the alloy must be dissolved by prolonged treatment with aqua regia, the solution evaporated to dryness, and the residue extracted with water. The solution thus obtained is treated with ammonic chloride in large excess and with some alcohol. A sparingly soluble[47] yellow ammonic platinum chloride is thrown down, mixed, perhaps, with the corresponding salts of other metals of the platinum group. Gold will be in solution. The solution is allowed to stand for some time, and then the precipitate is filtered off, washed with alcohol, dried, and transferred (wrapped in the filter paper) to a weighed crucible. It is ignited, gently at first, as there is danger of volatilising some of the platinum chloride, and afterwards intensely.

With large quant.i.ties of platinum the ignition should be performed in an atmosphere of hydrogen. Cool and weigh as metallic platinum.

IRIDIUM

Occurs in nature alloyed with osmium as osmiridium or iridosmine, which is "rather abundant in the auriferous beach sands of Northern California" (Dana). It occurs in bright metallic scales, which do not alloy with lead, and are insoluble in aqua regia. Iridium also occurs in most platinum ores, and forms as much as two per cent. of some commercial platinum. In chemical properties it resembles platinum, but the ammonic irido-chloride has a dark red colour, and on ignition leaves metallic iridium, which does not dissolve in aqua regia diluted with four or five times its volume of water and heated to a temperature of 40 or 50 C.

The other metals of the platinum group are Palladium, Rhodium, Osmium, and Ruthenium. They differ from gold, platinum, and iridium by the insolubility of their sulphides in a solution of sodium sulphide.

Palladium is distinguished by the insolubility of its iodide; and Osmium by the volatility of its oxide on boiling with nitric acid.

MERCURY.

Mercury occurs native and, occasionally, alloyed with gold or silver in natural amalgams; but its chief ore is the sulphide, cinnabar. It is comparatively rare, being mined for only in a few districts. It is chiefly used in the extraction of gold and silver from their ores (amalgamation); for silvering mirrors, &c.

Mercury forms two series of salts, mercurous and mercuric, but for the purposes of the a.s.sayer the most important property is the ease with which it can be reduced to the metallic state from either of these.

Mercury itself is soluble in nitric acid, forming, when the acid is hot and strong, mercuric nitrate. Cinnabar is soluble only in aqua regia.

Mercurous salts are generally insoluble, and may be converted into mercuric salts by prolonged boiling with oxidising agents (nitric acid or aqua regia). The salts of mercury are volatile, and, if heated with a reducing agent or some body capable of fixing the acid, metallic mercury is given off, which may be condensed and collected.

Mercury is separated from its solutions by zinc or copper, or it may be thrown down by stannous chloride, which, when in excess, gives a grey powder of metallic mercury, or, if dilute, a white crystalline precipitate of mercurous chloride. Nitric acid solutions of mercury yield the metal on electrolysis; and, if the pole on which the metal comes down be made of gold or copper, or is coated with these, the separated mercury will adhere thereto. It may then be washed and weighed.

The best tests for mercury next to obtaining globules of the metal are: (1) a black precipitate with sulphuretted hydrogen from acid solutions, which is insoluble in nitric acid; and (2) a white precipitate with stannous chloride.

DRY METHOD.

[Ill.u.s.tration: FIG. 47.]

Weigh up 5 grams, if the ore is rich, or 10 grams, if a poorer mineral.

Take a piece of combustion tube from 18 inches to 2 feet long, closed at one end, and place in it some powdered magnesite, so as to fill it to a depth of 2 or 3 inches, and on that a layer of an equal quant.i.ty of powdered lime (not slaked). Mix the weighed sample of ore in a mortar with 10 grams of finely powdered lime and transfer to the tube; rinse out the mortar with a little more lime, and add the rinsings. Cover with a layer of six or seven inches more lime and a loosely fitting plug of asbestos. Draw out the tube before the blowpipe to the shape shown in fig. 47, avoiding the formation of a ridge or hollow at the bend which might collect the mercury. Tap gently, holding the tube nearly horizontal, so as to allow sufficient s.p.a.ce above the mixture for the pa.s.sage of the gases and vapours which are formed. Place the tube in a "tube furnace," and, when in position, place a small beaker of water so that it shall just close the opening of the tube. The point of the tube should not more than touch the surface of the water. Bring the tube gradually to a red heat, commencing by heating the lime just behind the asbestos plug, and travelling slowly backwards. When the portion of the tube containing the ore has been heated to redness for some time the heat is carried back to the end of the tube. The magnesite readily gives up carbonic acid, which fills the tube and sweeps the mercury vapour before it. Some of the mercury will have dropped into the beaker, and some will remain as drops adhering to the upper part of the neck. Whilst the tube is still hot cut off the neck of the tube just in front of the asbestos plug (a drop of water from the wash bottle will do this), and wash the mercury from the neck into the beaker. The mercury easily collects into a globule, which must be transferred, after decanting off the bulk of the water, to a weighed Berlin crucible. The water is removed from the crucible, first by the help of filter paper, and then by exposing in a desiccator over sulphuric acid, where it should be left until its weight remains constant. It should not be warmed.

_Example_:--5 grams of an ore treated in this way gave 4.265 grams of mercury, equivalent to 85.3 per cent. Pure cinnabar contains 86.2 per cent.

WET METHODS.

_Solution._--Since solutions of chloride of mercury cannot be boiled without risk of loss,[48] nitric acid solutions should be used wherever possible. No mercury-containing minerals are insoluble in acids; but cinnabar requires aqua regia for solution. In dissolving this mineral nitric acid should be used, with just as much hydrochloric acid as will suffice to take it up.

To separate the mercury, pa.s.s sulphuretted hydrogen in considerable excess through the somewhat dilute solution. The precipitate should be black, although it comes down at first very light coloured. It is filtered, washed, and transferred back to the beaker, and then digested with warm ammonic sulphide. The residue, filtered, washed, and boiled with dilute nitric acid, will, in the absence of much lead, be pure mercuric sulphide. If much lead is present, a portion may be precipitated as sulphate, but can be removed by was.h.i.+ng with ammonic acetate. To get the mercury into solution, cover with nitric acid and a few drops of hydrochloric, and warm till solution is effected. Dilute with water to 50 or 100 c.c.

GRAVIMETRIC DETERMINATION.

This may be made by _electrolysis_. The same apparatus as is used for the electrolytic copper a.s.say may be employed, but instead of a cylinder of platinum one cut out of sheet copper should be taken, or the platinum one may be coated with an evenly deposited layer of copper. Fix the spiral and weighed copper cylinder in position, couple up the battery, _and when this has been done_ put the nitric acid solution of the mercury in its place.[49] The student had better refer to the description of the _Electrolytic Copper a.s.say_.

The mercury comes down readily, and the precipitation is complete in a few hours: it is better to leave it overnight to make sure of complete reduction. Disconnect the apparatus, and wash the cylinder, first with cold water, then with alcohol. Dry by placing in the water oven for two or three minutes. Cool and weigh: the increase in weight gives the amount of metallic mercury.

It must be remembered that copper will precipitate mercury without the aid of the battery; but in this case copper will go into solution with a consequent loss in the weight of the cylinder: this must be avoided by connecting the battery before immersing the electrodes in the a.s.say solution. The electrolysed solution should be treated with an excess of ammonia, when a blue coloration will indicate copper, in which case the electrolysis is unsatisfactory. With a little care this need not happen.

Gold cylinders may preferably be used instead of copper; but on platinum the deposit of mercury is grey and non-adherent, so that it cannot be washed and weighed.

VOLUMETRIC METHODS.

Several methods have been devised: for the details of these the student is referred to Sutton's "Handbook of Volumetric a.n.a.lysis."

QUESTIONS.

1. The specific gravity of mercury is 13.596. What volume would 8 grams occupy?

2. If 3.169 grams of cinnabar gave 2.718 grams of mercury, what would be the percentage of the metal in the ore?

3. Pour solution of mercuric chloride on mercury and explain what happens.

4. On dissolving 0.3 gram of mercury in hot nitric acid, and pa.s.sing sulphuretted hydrogen in excess through the diluted solution, what weight of precipitate will be got?