A Text-book of Assaying: For the Use of Those Connected with Mines Part 27

In the electrolytic a.s.say of metals, alloys, precipitates, and other bodies rich in copper, the preliminary separation of the copper by sulphuretted hydrogen is unnecessary. It is sufficient to dissolve the weighed sample in 10 c.c. of nitric acid, boil off nitrous fumes, dilute to 100 c.c. with water, and then electrolyse.

~General Considerations.~--In the preliminary work with the copper sulphide there is a small loss owing to its imperfect removal in was.h.i.+ng the filter paper, and another small loss in dissolving in nitric acid owing to the retention of particles in the fused globules of sulphur. To determine its amount the filter-papers and sulphur were collected from forty a.s.says, and the copper in them determined. The average amount of copper in each a.s.say was 0.175 gram; that left on the filter paper was 0.00067 gram; and that retained by the sulphur 0.00003 gram; thus showing an average loss from both sources of 0.00070 gram. The determinations from another lot of forty-two similar a.s.says gave on an average

Copper left on filter paper 0.00070 gram Copper retained by sulphur. 0.00004 "

The loss from these sources is trifling, and need only be considered when great accuracy is required.

The deposition of the copper under the conditions given is satisfactory, but, as already stated, if the solution contain more than 10 per cent.

of nitric acid it is not thrown down at all; or if a stronger current is used, say that from three Bunsen cells, it will be precipitated in an arborescent brittle form, ill adapted for weighing. It may be noted here that increasing the size of the cells does not necessarily increase the intensity of the current.

In two determinations on pure electrotype copper the following results were obtained:--

Copper Taken. Copper Found.

0.8988 gram 0.8985 gram 0.8305 " 0.8303 "

The presence of salts of ammonia, &c., somewhat r.e.t.a.r.ds the deposition, but has no other ill effect.

The organic matter generally present in copper ores interferes more especially in the colorimetric determination of the residual copper. It can be detected on dissolving the ore as a light black residue insoluble in nitric acid. It is filtered off at once, or, if only present in small amount, it is carried on in the ordinary process of the a.s.say and separated in the last filtration before electrolysis.

The following experiments were made to test the effect of the presence of salts of foreign metals in the solution during the precipitation of copper by electrolysis:--

--------------+----------------------------------------+--------------- Copper Taken. | Other Metal Added. | Copper Found.

--------------+----------------------------------------+--------------- 0.1000 gram | 0.1000 gram of silver | 0.1800 0.1050 " | 0.1000 " " | 0.2000 0.1030 " | 0.1000 " mercury | 0.2010 0.1037 " | 0.1000 " " | 0.2015 0.1020 " | 0.1000 " lead | 0.1020 0.1030 " | 0.1000 " " | 0.1028 0.1010 " | 0.1000 " a.r.s.enic | 0.1010 0.1007 " | 0.1000 " " | 0.1022 0.1030 " | 0.1000 " antimony | 0.1050 0.1034 " | 0.1000 " " | 0.1057 0.0990 " | 0.1200 " tin | 0.0990 0.1014 " | 0.1000 " " | 0.1015 0.1000 " | 0.1000 " bis.m.u.th | 0.1662 0.1040 " | 0.1000 " cadmium | 0.1052 0.1009 " | 0.1300 " zinc | 0.1017 0.1014 " | 0.1000 " nickel | 0.1007 0.1079 " | 0.1200 " iron | 0.1089 0.1054 " | 0.1000 " chromium (Cr_{2}O_{3}) | 0.1035 0.1034 " | 0.1000 " " (K_{2}CrO_{4}) | 0.1010 0.1075 " | 0.1000 " aluminium | 0.1078 0.1010 " | 0.1000 " manganese | 0.0980 --------------+----------------------------------------+---------------

It will be seen from these that mercury, silver, and bis.m.u.th are the only metals which are precipitable[52] along with the copper under the conditions of the a.s.say. Mercury, which if present would interfere, is separated because of the insolubility of its sulphide in nitric acid.

Bis.m.u.th is precipitated only after the main portion of the copper is thrown down. It renders the copper obviously unsuitable for weighing. It darkens, or forms a greyish coating on, the copper; and this darkening is a delicate test for bis.m.u.th. In a.s.saying ores containing about three and a half per cent. of copper, and known to contain bis.m.u.th in quant.i.ties scarcely detectable in ordinary a.n.a.lysis, the metal deposited was distinctly greyish in colour, and would not be mistaken for pure copper. Ten grams of this impure copper were collected and a.n.a.lysed, with the following results:--

Copper 99.46 per cent.

Bis.m.u.th 00.30 "

Iron 00.14 "

a.r.s.enic 00.10 "

------ 100.00

The quant.i.ty of copper got in each a.s.say was 0.175 gram, and consequently the bis.m.u.th averaged 0.00053 gram.

To separate the bis.m.u.th in such a case the deposit is dissolved off by warming it in the original solution. The bis.m.u.th is precipitated by the addition of ammonic carbonate, and the solution, after filtering and acidifying with nitric acid, is re-electrolysed.

~Determination of Copper in Commercial Copper.~--Take from 1 to 1.5 gram, weigh carefully, and transfer to a beaker; add 20 c.c. of water and 10 c.c. of nitric acid; cover with a clock gla.s.s, and allow to dissolve with moderate action; boil off nitrous fumes, dilute to 100 c.c., and electrolyse. The cylinder must be carefully weighed, and the electrolysis allowed to proceed for 24 hours. The weight found will be that of the copper and silver. The silver in it must be determined[53]

and deducted.

~Determination of Copper in Bra.s.s, German Silver, or Bronze.~--Treat in the same manner as commercial copper. If nickel is present, the few milligrams of copper remaining in the electrolysed solution should be separated with sulphuretted hydrogen, the precipitated sulphide dissolved in nitric acid, and determined colorimetrically.

VOLUMETRIC PROCESSES.

There are two of these in use, one based on the decolorising effect of pota.s.sic cyanide upon an ammoniacal copper solution, and the other upon the measurement of the quant.i.ty of iodine liberated from pota.s.sic iodide by the copper salt. The cyanide process is the more generally used, and when carefully worked, "on certain understood and orthodox conditions,"

yields good results; but probably there is no method of a.s.saying where a slight deviation from these conditions so surely leads to error. An operator has no difficulty in getting concordant results with duplicate a.s.says; yet different a.s.sayers, working, without bias, on the same material, get results uniformly higher or lower; a difference evidently due to variations in the mode of working. Where a large number of results are wanted quickly it is a very convenient method. The iodide process is very satisfactory when worked under the proper conditions.

CYANIDE METHOD.

The process is based upon the facts--(1) that when ammonia is added in excess to a solution containing cupric salts, ammoniacal copper compounds are formed which give to the solution a deep blue colour; and (2) that when pota.s.sic cyanide is added in sufficient quant.i.ty to such a solution the colour is removed, double cyanides of copper and pota.s.sium or ammonium being formed.[54] In the explanation generally given the formation of cuprous cyanide is supposed[55]; but in practice it is found that one part of copper requires rather more than four parts of cyanide, which agrees with the former, rather than the latter, explanation.

Reliance on the accuracy of the process cannot rest upon the supposition that the cyanide required for decoloration is proportional to the copper present, for varying quant.i.ties of ammonia salts, ammonia and water, and differences of temperature have an important effect. The results are concordant and exact only when the cyanide is standardised under the same conditions as it is used. It is best to have the a.s.say solution and that used for standardising as nearly as possible alike, and to t.i.trate the two solutions side by side. This demands an approximate knowledge of the quant.i.ty of copper contained in the ore and a separation of the bulk of the impurities.

For the t.i.tration there is required a standard solution of pota.s.sium cyanide made by dissolving 42 grams of the salt, known to dealers as Pota.s.sium Cyanide (Gold), in water and diluting to one litre: 100 c.c.

of this will be about equivalent to one gram of copper. For poor ores the solution may conveniently be made half this strength.

The solution of the ore and the separation of the copper as sulphide are effected in the same ways as have been already described for electrolysis. Similarly, too, the sulphide is attacked with 15 c.c. of nitric acid and the a.s.say boiled down to 10 c.c. Add 20 c.c. of water and warm, filter into a pint flask, wash well with water, and dilute to about 150 c.c.; add 30 c.c. of dilute ammonia, and cool.

Prepare a standard by dissolving a quant.i.ty of electrotype copper (judged to be about the same as that contained in the a.s.say) in 20 c.c.

of water and 10 c.c. of nitric acid, boil off the nitrous fumes, and dilute to 150 c.c.: add 30 c.c. of dilute ammonia and cool.

Fill a burette with the standard cyanide solution. The burette with syphon arrangement, figured on page 52, is used. A number of t.i.trations can be carried on at the same time provided the quant.i.ty of copper present in each is about the same. This is regulated in weighing up the ore. The flasks must of course be marked, and should be arranged in series on a bench in front of a good light and at such a height that the liquid can be looked through without stooping. Supposing about 50 c.c.

of cyanide will be required, 30 c.c. should be run into each, and each addition be recorded as soon as made; then run 15 c.c. into each. The solutions will now probably show marked differences of tint: add 1 c.c.

of cyanide to the lighter ones and more to the darker, so as to bring the colours to about the same depth of tint. They should all be of nearly equal tint just before finis.h.i.+ng. At the end add half a c.c. at a time until the colours are completely discharged. A piece of damp filter paper held between the light and the flask a.s.sists in judging the colour when nearly finished. Overdone a.s.says show a straw yellow colour which deepens on standing.

The following will ill.u.s.trate the notes recorded of five such a.s.says and one standard:--

(1) 30 c.c. 15 c.c. 5 c.c. 2 c.c. 1 c.c. 1/2 c.c. -- c.c. = 53-1/2 c.c.

(2) 30 " 15 " 1 " 1 " 1 " 1/2 " -- " = 48-1/2 "

(3) 30 " 15 " 3 " 1 " 1 " 1/2 " -- " = 50-1/2 "

(4) 30 " 15 " 5 " 2 " 1 " 1/2 " 1/2 " = 54 "

(5) 30 " 15 " 2 " 1 " 1 " 1/2 " -- " = 49-1/2 "

(6) 30 " 15 " 2 " 1 " 1 " 1/2 " 1/2 " = 50 standard

Three grams of ore were taken, and the standard contained 0.480 gram of copper.

In this series the difference of half a c.c. means about 0.15 per cent.

on the ore; with a little practice it is easy to estimate whether the whole or half of the last addition should be counted.

To get satisfactory results, the manner of finis.h.i.+ng once adopted must be adhered to.

The following experiments show the effect of variation in the conditions of the a.s.say:--Use _a solution of copper nitrate_, made by dissolving 10 grams of copper in 50 c.c. of water and 35 c.c. of nitric acid, and diluting to a litre. 100 c.c. = 1 gram of copper.

~Effect of Varying Temperature.~--In these experiments 20 c.c. of copper nitrate were used, with 10 c.c. of nitric acid, 30 c.c. of dilute ammonia, and water to 200 c.c. The results were--

Temperature 15 30 70 100 Cyanide required 21.5 c.c. 20.8 c.c. 19.7 c.c. 18.8 c.c.

The temperature is that of the solution _before_ t.i.trating. These show the importance of always cooling before t.i.trating, and of t.i.trating the a.s.say and standard at the same temperature.

~Effect of Varying Bulk.~--The quant.i.ties of copper, acid, and ammonia were the same as in the last-mentioned experiments. The results were:--

Bulk 100.0 c.c. 200.0 c.c. 300.0 c.c. 400.0 c.c.

Cyanide required 23.3 " 21.7 " 21.4 " 21.4 "

These show that large variations in bulk must be avoided.