A Text-book of Assaying: For the Use of Those Connected with Mines Part 30

~Gold.~--To determine it, dissolve 10, 20, or 50 grams of the sample in 35, 70, or 175 c.c. of nitric acid and an equal volume of water, boil till free from nitrous fumes, and dilute to double its volume. Allow to stand for some time, decant on to a filter, dry, burn, and cupel the ashes with 1 gram of sheet lead. If silver is present, owing to traces of chlorides in the re-agents used, "parting" will be necessary. (See _Gold_.)

Working in this way on 20 grams of copper, to which 1.8 milligram of gold had been added, a b.u.t.ton weighing 2.0 milligrams was obtained.

~Antimony~ is not a frequent impurity of copper: it can be detected in quant.i.ties over 0.1 per cent. by a white residue of Sb_{2}O_{4}, insoluble in nitric acid. With material containing only small quant.i.ties of antimony the white oxide does not show itself for some time, but on long-continued boiling it separates as a fine powder. It is best (when looking for it) to evaporate the nitric acid solution to the crystallising point, to add a little fresh nitric acid and water, and then to filter off the precipitate. After weighing it should be examined for a.r.s.enic and bis.m.u.th.

~Lead.~--Refined coppers are often free from lead, anything more than traces being seldom found; in coa.r.s.e coppers it is sometimes present in considerable quant.i.ties.

Its presence may be detected in the estimation of the copper electrolytically, the platinum spiral becoming coated with a brown or black deposit of lead dioxide. The depth of colour varies with the lead present, and obviously could be made the basis of an approximate estimation. The colour shows itself within an hour or so, but is best observed when all the copper has been deposited.

Electrolysing a solution of one gram of pure copper, to which 0.5 milligram of lead had been added, the deposit was dark brown; in a similar solution with 1 milligram of lead it was much darker, and with 2 milligrams it was black. Under the conditions of the a.s.say the dioxide cannot be weighed, as it partly dissolves on breaking the current. When lead has been found, its quant.i.ty may be estimated by evaporating to dryness the nitric acid solution to which an excess of sulphuric acid has been added, taking up with water, and filtering off and weighing the lead sulphate.

The separation of traces of lead as chromate is a fairly good one.

Dissolve 5 grams of the copper in 17 c.c. of nitric acid and an equal volume of water; boil off nitrous fumes, neutralise with soda, and afterwards acidulate with acetic acid; and dilute to a litre. Add 20 grams of sodium acetate, warm, and precipitate the lead with a dilute solution of pota.s.sium chromate. Copper chromate (yellow) may be at the same time thrown down, but it is readily soluble on diluting. Filter off the precipitate; wash it into a beaker and pa.s.s sulphuretted hydrogen; oxidise the sulphide and weigh as lead sulphate. Treated in this way 5 grams of copper yielded sulphate of lead equal to 2.0 milligrams of lead. Five grams of the same sample to which 10 milligrams of lead were added gave 11.4 milligrams.

~Nickel and Cobalt.~--Nickel is always present in larger or smaller quant.i.ties in commercial copper, and, perhaps, has an influence on the properties of the metal. It is determined as follows:--Dissolve 10 grams of the copper in 35 c.c. of nitric acid and an equal bulk of water, boil off nitrous fumes and neutralise with soda, add 2 grams of carbonate of soda dissolved in water, boil, and filter. Acidify the filtrate with 2 or 3 c.c. of dilute nitric acid and dilute to 1 or 1-1/2 litres. Pa.s.s sulphuretted hydrogen through the cold solution till the copper is all down and the liquid smells of the gas. Filter and evaporate the filtrate to a small bulk, and determine the nickel by electrolysing the solution rendered ammoniacal, or by precipitating as sulphide and weighing as sulphate. (See under _Nickel_.) The precipitate, after weighing, should be tested for cobalt. If present it is separated with pota.s.sium nitrite as described under _Cobalt_. Ten grams of copper gave 6.0 milligrams of nickel; and another lot of 10 grams of the same copper, to which 10.0 milligrams of nickel had been added, gave 17.2 milligrams.

~Sulphur.~--The amount of sulphur in refined copper is very small, seldom exceeding 0.005 per cent. In coa.r.s.e copper, as might be expected, it is found in larger quant.i.ties.

In determining it, it is first converted into sulphuric acid, and then precipitated and weighed as barium sulphate. The precipitation cannot be effected from a nitric acid solution. Ten grams of copper are dissolved in nitric acid, as for the other determinations, and then boiled with excess of hydrochloric acid till the nitric acid is completely removed.

There is then added a few drops of a dilute solution of baric chloride, and the solution is allowed to stand for some hours. The baric sulphate is filtered off and weighed.

The necessity for precipitating from a hydrochloric acid solution is seen from the following determinations. In each experiment 10 grams of copper was used, and a known weight of sulphur, in the form of copper sulphate, added.

--------------+---------------------------+--------------------- Sulphur added.| Sulphur found in | Sulphur found in |Hydrochloric Acid Solution.|Nitric Acid Solution.

--------------+---------------------------+--------------------- 5 milligrams | 8 milligrams | 0.03 milligrams 10 " | 11 " | 0.03 "

15 " | 17 " | 12.00 "

~Bis.m.u.th.~--Nearly all samples of copper contain bis.m.u.th, but only in small quant.i.ties. It is best determined colorimetrically as described under _Bis.m.u.th_. The method of concentrating and preparing the solution for colorimetric a.s.say is as follows. Dissolve 10 grams of copper in nitric acid, as in the other determinations; neutralise with soda; add 1 or 1.5 grams of bicarbonate of soda and boil for ten minutes; filter, dissolve the precipitate in hot dilute sulphuric acid; add sulphurous acid and pota.s.sium iodide in excess, and boil till free from iodine.

Filter and dilute to 500 c.c. Take 50 c.c. of the yellow solution for the determination. A few c.c. of a dilute solution of sulphurous acid (1 in 100) will prevent the liberation of iodine. The following experiments test the method of separation. Ten grams of copper were treated as above and precipitated with 1.5 gram of "soda;" the precipitate contained 0.6 milligram of bis.m.u.th (= 0.006 per cent.). The filtrate treated with another 1.5 gram of "soda" gave a precipitate which was free from bis.m.u.th. To the filtrate from this was added 1.0 milligram of bis.m.u.th, and another fraction was precipitated with 1.5 gram of "soda." In this precipitate was found 1.0 milligram of bis.m.u.th. To the filtrate another milligram of bis.m.u.th was added and the separation with "soda" repeated.

The bis.m.u.th was separated from this precipitate with ammonic carbonate before determination, and 0.9 milligram was found.

~a.r.s.enic.~--The proportion of a.r.s.enic in copper varies from 0.01 to 0.75 per cent. whilst in coa.r.s.e copper it may amount to 2 or even 3 per cent.

To determine it, dissolve 5, 10, or 20 grams of the copper (according to the amount of a.r.s.enic present) in 18 c.c., 35 c.c., or 70 c.c. of nitric acid, and an equal volume of water. Boil off the nitrous fumes, dilute to 100 c.c. and neutralise with soda; add 1.5 or 2 grams of carbonate of soda dissolved in a little water, and boil. Filter (was.h.i.+ng is unnecessary) and dissolve back into the flask with a little dilute hydrochloric acid; add 30 c.c. of dilute ammonia and 25 c.c. of "magnesia mixture," and allow to stand overnight. The whole of the a.r.s.enic is precipitated as ammonic-magnesic a.r.s.enate in one hour, but it is advisable to leave it longer. The precipitate may be dried and weighed, or, better, t.i.trated with uranium acetate. (See _a.r.s.enic_.) To test this method of separation 10 grams of pure copper were taken and 0.200 gram of a.r.s.enic dissolved with it. The a.r.s.enic was determined by t.i.tration with uranium acetate, and 0.200 gram was found. Two other similar experiments with 0.080 and 0.010 gram of a.r.s.enic added, gave 0.079 and 0.012 gram respectively.

Antimony or bis.m.u.th may be present without interfering with the t.i.tration. With 0.100 gram of antimony and 0.100 gram of a.r.s.enic, 0.100 gram of a.r.s.enic was found; and in another case, with 0.100 gram of bis.m.u.th and 0.060 gram of a.r.s.enic, 0.060 gram was found. In these experiments the antimony and bis.m.u.th were present in the a.s.say solution when t.i.trated. For a gravimetric determination they would require to be removed before precipitating with "magnesia mixture."

Phosphorus, if present, counts as a.r.s.enic in the proportion of 1 to 2.4; but, except in the case of coa.r.s.e coppers, it is always absent.

Iron, if present, interferes by forming a white flocculent precipitate of ferric a.r.s.enate after the addition of the sodium acetate and preliminary to the t.i.tration. Each milligram of iron abstracts, in this way, 1.3 milligrams of a.r.s.enic.

~Iron.~--Refined coppers carry traces of iron, varying from 0.001 to 0.01 per cent. It is best determined during the a.r.s.enic estimation. The precipitate of the ammonic-magnesic a.r.s.enate will contain the whole of the iron as ferric hydrate. On dissolving in hydrochloric acid, neutralising with ammonia, adding 5 c.c. of sodic acetate, diluting, and boiling, it reappears as a white precipitate of ferric a.r.s.enate. It is filtered off (the a.r.s.enic being estimated in the filtrate), dissolved in warm hydrochloric acid, and determined colorimetrically as described under _Iron_. A series of experiments testing the separation is there given.

~Phosphorus.~--Refined coppers do not carry phosphorus, although it may be present in "coa.r.s.e copper" up to 1 per cent. or more. In such samples the following method is adopted for the estimation of both phosphorus and a.r.s.enic. Dissolve 10 grams of copper and 0.1, 0.2, or 0.3 gram of iron wire (according to the amount of a.r.s.enic and phosphorus present) in 35 c.c. of nitric acid and an equal volume of water. Add soda till the free acid is nearly neutralised. Next add a strong solution of sodium acetate, until the solution ceases to darken on further addition, then dilute with water to half a litre. The solution is best contained in a large beaker; it is next heated to the boiling point, and at once removed and allowed to settle. If the precipitate is light coloured it is evidence that sufficient iron has not been added, or, if it is green, from basic copper salts, it shows that the solution was not sufficiently acid. In either case start afresh. Filter off the precipitate and wash with hot water containing a little sodium acetate, dissolve it off the filter with hot dilute hydrochloric acid, add ammonia in excess, and pa.s.s sulphuretted hydrogen for five minutes. Warm at about 70 C. for a quarter of an hour. Filter. The clear yellow filtrate contains the a.r.s.enic and phosphorus. Add dilute sulphuric acid in excess; filter off the yellow precipitate of sulphide of a.r.s.enic, dissolve it in nitric acid, and t.i.trate with uranium acetate, as described under _a.r.s.enic_.

The filtrate from the sulphide of a.r.s.enic is rendered alkaline with ammonia and "magnesia mixture" added. The solution is stirred, and allowed to stand overnight. The precipitate of ammonic-magnesic phosphate is filtered off, dissolved, and t.i.trated with uranium acetate, using the same standard solution as is used in the a.r.s.enic a.s.say: 0.5 gram of a.r.s.enic equals 0.207 gram of phosphorus.

~Copper.~--The method of determining this has been described under _Electrolytic a.s.say_.

In the method of concentration by fractional precipitation with sodic carbonate (which is adopted in most of these determinations) the precipitate will contain all the bis.m.u.th, iron, and alumina; the a.r.s.enic and phosphorus as cupric a.r.s.enate and phosphate; and the greater part of the lead, antimony, and silver. The nickel and cobalt, and the sulphur as sulphuric acid, will remain in solution with the greater part of the copper.

PRACTICAL EXERCISES.

1. According to a wet a.s.say 2 grams of a certain ore contained 0.3650 gram of copper. What would you expect the dry a.s.say produce to be?

2. A standard solution is made by dissolving 25 grams of pota.s.sic cyanide and diluting to a litre. a.s.suming the salt to be 98 per cent.

real cyanide, what would 100 c.c. of the solution be equivalent to in grams of copper?

3. How would you make a solution of "hypo" of such strength that 100 c.c. shall equal 0.633 gram of copper?

4. What weight of ore, containing 17.0 per cent. of copper, would you take in order to get about 0.5 gram of copper in solution for electrolysis?

5. The solution of copper in nitric acid is effected by the following reaction:--

3Cu + 8HNO_{3} = 3Cu(NO_{3})_{2} + 4H_{2}O + 2NO.

What volume of nitric acid will be required to dissolve 1 gram of copper?

LEAD.

The chief ore of lead is galena, a sulphide of lead, common in most mining districts, and frequently a.s.sociated with blende and copper-pyrites. It always carries more or less silver; so that in the a.s.say of the ore a silver determination is always necessary. Carbonate (cerussite), sulphate (anglesite), and phosphate (pyromorphite) of lead also occur as ores, but in much smaller quant.i.ties.

Lead ores are easily concentrated (owing to their high specific gravity, &c.) by mechanical operations, so that the mineral matter sent to the smelter is comparatively pure.

Lead is readily soluble in dilute nitric acid. The addition of sulphuric acid to this solution throws down heavy, white, and insoluble lead sulphate.

Galena is soluble in hot hydrochloric acid, sulphuretted hydrogen being evolved; but the action is r.e.t.a.r.ded by the separation of the sparingly soluble lead chloride. If a rod of zinc is placed in this solution, metallic lead is precipitated on it as a spongy ma.s.s, the lead chloride being decomposed as fast as it is formed. The opening up of the ore is thus easily effected, the sulphur going off as sulphuretted hydrogen, and the lead remaining in a form easily soluble in dilute nitric acid.

Galena itself is readily attacked by nitric acid, part of the lead going into solution, and the rest remaining as insoluble lead sulphate. The sulphate is due to the oxidation of the sulphur by nitric acid; its amount will vary with the quant.i.ty and concentration of the acid used.

Sulphate of lead is soluble in solutions of ammonium or sodium acetate; or it may be converted into carbonate by boiling with carbonate of soda.

The carbonate, after was.h.i.+ng off the sulphate of soda, dissolves easily in nitric acid. The precipitation of lead from acid solutions with sulphuric acid, and the solubility of the precipitate in ammonium acetate, distinguishes it from all other metals. The addition of pota.s.sium chromate to the acetate solution reprecipitates the lead as a yellow chromate.

DRY a.s.sAY.

The dry a.s.say of lead is largely used, but it is only applicable to rich or concentrated ores, and even with these only gives approximate results. Both lead and lead sulphide are sensibly volatile at a moderately-high temperature; hence it is necessary to obtain a slag which is easily fusible. As a reducing agent iron is almost always used, and this is added either in the form of an iron rod, or the crucible itself is made of this metal. The flux used is carbonate of soda.

When a clay crucible is used, the method of working is as follows:--Weigh up 25 grams of the dry and powdered ore, mix with an equal weight of "soda" and 2 grams of tartar; place in a crucible (E.

Battersea round), and then insert a piece of iron rod about half an inch in diameter, and of such a length that it will just allow the crucible to be covered. The rod should be pushed down so as to touch the bottom of the crucible, and the mixture should be covered with a sprinkling of borax. Place in a furnace heated to, but not above, redness, and cover the crucible. In about twenty minutes the charge will be fused: the fusion is complete when bubbles of gas are no longer being evolved; and then, but not till then, the iron is withdrawn, any adhering b.u.t.tons of lead being washed off by dipping the rod a few times in the slag. Cover the crucible, leave it for a minute or two, and then pour. Detach the slag, when cold, by hammering. The weight of the b.u.t.ton multiplied by 4 gives the percentage. The commoner errors of students in working the process are too high a temperature and too quick a withdrawal.

A sample of ore treated in this manner gave on duplicate a.s.say 17.5 and 17.6 grams of lead, equalling 70.0 and 70.4 per cent. respectively. By wet a.s.say the sample gave 73.3 per cent. Using an iron crucible, the results will be 1 per cent. or so higher. The crucible must be made of wrought iron; and, if it has been previously used, should be cleaned by heating to dull redness and sc.r.a.ping the scale off with a stirrer. Take 30 grams of the ore, mix with 30 grams of "soda" and 3 grams of tartar; put the mixture in the crucible, and cover with a sprinkling of borax; heat for about twenty minutes at not too high a temperature, and then sc.r.a.pe down the slag adhering to the side with a stirrer. Leave in the furnace till action has ceased. Before pouring, tap the pot gently, and then tilt it so as to make the slag wash over the part of the crucible along which the charge is to be poured. Pour; and, when cold, clean and weigh the b.u.t.ton of metal. A crucible may be used from ten to twenty times.

These a.s.says are for ores containing the lead chiefly as sulphide. For oxidised ores, charcoal or tartar is employed as the reducing agent. The student may practise on red lead as follows:--Take 30 grams of red lead; mix with 10 grams each of borax and "soda" and about 1.5 gram of powdered charcoal; place in a small clay crucible with a cover (C.

Battersea round), fuse at a gentle heat, and pour when action ceases.

This a.s.say will only take a few minutes.

Where lead is present as phosphate (as in the case of pyromorphite), or mixed with phosphates (as sometimes happens), carbonate of soda is a suitable flux; but the phosphate of soda which is formed makes a thick tenacious slag, which is very apt to be carried out of the pot by the escaping gas. A wide-mouthed clay pot is taken and a little fluor spar added. For the a.s.say of pyromorphite the following charge may be used:--Ore, 20 grams; "soda," 25 grams; tartar, 7 grams; and fluor spar, 5 grams; and 2 grams of borax as a cover. This will melt down in about ten minutes, and should be poured as soon as tranquil.