A Text-book of Assaying: For the Use of Those Connected with Mines Part 46

Compounds of manganese, on boiling with strong hydrochloric acid, yield manganous chloride[80] (MnCl_{2}).

The properties given above serve for the detection of manganese; the higher oxides are distinguished by causing the evolution of chlorine (with its peculiarly suffocating smell) when acted on with hydrochloric acid; while the green "melt," with sodium carbonate, can be relied on for the recognition of manganese itself. There is no dry a.s.say of manganese ores.

WET METHODS.

Strong hydrochloric acid is the best solvent for ores of manganese; but where the proportion of dioxide (MnO_{2}) is required, the solution is effected during the a.s.say. The ore should be in a very fine state of division before treatment with acids.

The separation of manganese from other metals is thus effected: Ignite, in order to destroy any organic matter which may be present; dissolve in hydrochloric acid, and evaporate to dryness, to separate silica. Take up with hydrochloric acid, dilute, pa.s.s sulphuretted hydrogen, and filter.

Boil off the excess of gas, peroxidise the iron with a drop or two of nitric acid, and separate the iron as basic acetate (as described under _Iron_).[81] If the iron precipitate is bulky, it is dissolved in a little hydrochloric acid, reprecipitated, and the filtrate added to the original one. Neutralise with soda, and add bromine in excess; heat gradually to boiling, allow to settle, and filter. The precipitate is impure dioxide of manganese (containing alkalies and, possibly, cobalt or nickel).

GRAVIMETRIC DETERMINATION.

Dissolve the precipitate in hydrochloric acid, and boil; add a slight excess of carbonate of soda, warm, and filter. Wash with hot water, dry, carefully ignite in an open Berlin crucible, and weigh. The substance is the brown oxide (Mn_{3}O_{4}), and contains 72.05 per cent. of manganese. If the percentage of dioxide is required it may be calculated by multiplying the percentage of manganese by 1.582. It must be borne in mind that the manganese should never be calculated to dioxide except when it is known to exist in the ore only in that form.

VOLUMETRIC METHODS.

The two methods are based on the oxidising effect of manganese dioxide; and if the metal does not already exist in this form it will require a preliminary treatment to convert it. The following method due to Mr. J.

Pattinson[82] effects this: A quant.i.ty of the ore containing not more than .25 grams of the metal (Mn), is dissolved in hydrochloric acid in a pint beaker, and, if necessary, 3 or 4 c.c. of nitric acid are added to peroxidise the iron, and ferric chloride is added if required, so that there may be at least as much iron as manganese. Calcium carbonate is added till the solution is slightly red; and next the redness is removed by the cautious addition of acid; 30 c.c. of zinc chloride solution (containing 15 grams of zinc per litre) are added, the liquid is brought to boil and diluted to about 300 c.c. with boiling water; 60 c.c. of a solution of bleaching powder (33 grams to the litre and filtered), rendered slightly greenish by acid, are then run in and are followed by 3 grams of calcium carbonate suspended in 15 c.c. of boiling water.

During effervescence the beaker is covered, the precipitate is stirred, and 2 c.c. of methylated spirit are mixed in. The precipitate is collected on a large filter, washed with cold water, and then with hot, till free from chlorine, which is tested for with starch and pota.s.sium iodide. The acid ferrous sulphate solution (presently described) is then measured into the beaker, and the precipitate, still in the paper, added; more acid is added (if necessary), and the solution is diluted and t.i.trated. In place of bleaching powder solution, 90 c.c. of bromine water (containing 22 grams per litre) may be used.

FERROUS SULPHATE a.s.sAY.

This method, which is the one commonly used, is based on the determination of the amount of ferrous iron oxidised by a known weight of the ore. It is known that 87 parts of the dioxide will oxidise 112 parts of ferrous iron;[83] therefore 1 gram will oxidise 1.287 gram of ferrous iron, or 1 gram of ferrous iron oxidised will be equivalent to 0.7768 gram of the dioxide. The finely-divided substance containing the dioxide is digested in a solution of a known quant.i.ty of iron in sulphuric acid. The iron, of course, must be in excess, which excess is determined when the ore is dissolved by t.i.trating with standard permanganate or b.i.+.c.hromate of potash solution. The a.s.say resolves itself into one for the determination of ferrous iron, for which the standard solutions and method of working described under _Iron_ are used.

The a.s.say is as follows:--For rich ores, 2 grams of clean soft iron wire are treated, in a pint flask, with 100 c.c. of dilute sulphuric acid and warmed till dissolved. Carefully sample the ore, and in one portion determine the "moisture at 100 C.;" grind the rest in a Wedgwood mortar with a little pure alcohol until free from grit. This reduces the substance to a finely-divided state and a.s.sists solution. Evaporate off the alcohol and dry at 100 C., mix well, and keep in a weighing-bottle.

Weigh up 2 grams and add them to the solution of iron in the flask; carefully wash it all down into the acid liquid. On rotating the flask the ore will rapidly dissolve, but gentle heat may be used towards the end to complete the solution. When the residue is clean and sandy-looking, and free from black particles, the flask is cooled, and the residual ferrous iron is determined by t.i.tration with "permanganate." The iron thus found, deducted from the 2 grams taken, will give the amount of iron peroxidised by the dioxide contained in the 2 grams of ore. This divided by 2 and multiplied by 77.68 will give the percentage of dioxide in the sample, or multiplied by 49.41 will give that of metallic manganese.

When the quant.i.ty of manganese or of the dioxide to be determined is small, it is not necessary to use 2 grams of iron; 1 gram, or even less, may be taken. The iron may be used in the form of a standard solution of ferrous sulphate and portions measured off, thus saving the labour of weighing.

~Determination of Dioxide in a Manganese Ore.~--Weigh up 1 or 2 grams of the finely-powdered ore[84] and an equal weight of pure iron wire, dissolve the wire in 50 or 100 c.c. of dilute sulphuric acid, and, when solution is complete, add the ore and warm till it too is dissolved.

Cool and t.i.trate the remaining ferrous iron with the permanganate or b.i.+.c.hromate of pota.s.sium solution.

For example, 0.7560 gram of pyrolusite and 1.000 gram of iron were taken and treated as above; 13.9 c.c. of "permanganate" (standard 100 c.c. = 0.4920 gram iron) were required; this indicates that 0.0684 gram of iron was left unoxidised by the ore. The iron oxidised, then, was 0.9316 gram (1.000 - 0.0684); multiplying this by 0.7768, we find that 0.7237 gram is the quant.i.ty of manganese dioxide which was present. This is equivalent to 95.77 per cent.;

0.7560 : 0.7237 :: 100 : 95.77.

IODINE METHOD.

It has been already stated that when dioxide of manganese is boiled with strong hydrochloric acid chlorine is given off, and that the amount of chlorine so liberated is a measure of the dioxide present. If the chlorine is pa.s.sed into a solution of pota.s.sium iodide, an equivalent of iodine will be set free.[85] This is apparently a very indirect way of determining how much of the dioxide is present; but the reactions are very sharp, and the final determination of the iodine is an easy one.

[Ill.u.s.tration: FIG. 60.]

The finely-powdered sample of dioxide is placed in a small flask provided with an exit tube leading into a solution of pota.s.sic iodide (fig. 60). On adding hydrochloric acid and boiling, the chlorine evolved is driven into the iodide solution and there absorbed; the boiling is continued till the steam and hydrochloric acid fumes have driven the last portions of the chlorine out of the flask and into the solution. In this experiment there is a strong tendency for the iodide solution to rush back into the flask. This tendency is overcome by avoiding draughts and regulating the heat; or by placing a lump of magnesite in the flask, which acts by evolving carbonic acid and so producing a steady outward pressure. When the distillation is finished the tube containing the iodine is detached and washed out into a beaker. If the solution is strongly acid it should be almost neutralised by the cautious addition of dilute ammonia. If crystals of iodine have separated, pota.s.sium iodide must be added in quant.i.ty sufficient to dissolve them. The condenser must be kept cool whilst the chlorine is pa.s.sing into it.

The solution, transferred to a beaker, is t.i.trated with a standard solution of sodic hyposulphite (100 c.c. = 1.27 gram iodine or 0.435 gram of dioxide of manganese). In t.i.trating, the solution should be cold, or not warmer than 30 C. The bulk may vary from 100 to 200 c.c.; but it is best always to work with the same volume. The "hypo" is run in with constant agitation until the brown colour has been reduced to a light yellow; 5 c.c. of starch solution are then added and the t.i.tration cautiously continued until the end is reached; the finish is indicated by a change from blue to colourless.

The a.s.say solution may be acidified with acetic, sulphuric, or hydrochloric acid before t.i.trating with "hypo;" but it must be only faintly so. An excess of acid may be nearly neutralised with ammonia without interference, but excess of alkali is fatal. Bicarbonate of soda must not be used in excess; it is best to avoid it altogether. The a.s.say solution should be t.i.trated at once, as it weakens on standing; and the "hypo" solution should be standardised every two or three days, as its strength is not constant.

_The standard solution of hyposulphite of soda_ is made by dissolving 25 grams of the salt (Na_{2}S_{2}O_{3}.5H_{2}O) in water and diluting to 1 litre. 100 c.c. are equivalent to 1.27 gram of iodine.

This solution is standardised by weighing, in a small beaker, about half a gram of iodine, to which is added a crystal or two of pota.s.sium iodide and a few drops of water. When dissolved, the solution is diluted to 100 c.c., and t.i.trated in the manner described. The starch solution is made in the manner described under the iodide copper a.s.say. 5 c.c. are used for each t.i.tration.

In determining the effects of variations in the condition of the a.s.say a solution of iodine was used, which was equivalent in strength to the "hypo" solution. It was made by dissolving 12.7 grams of iodine with 25 grams of pota.s.sium iodide in a little water and diluting to 1 litre. 100 c.c. of this solution were found (at the time of the experiments) to be equivalent to 102.0 c.c. of the "hypo."

~Effect of Varying Temperature.~--The bulk of the solution was 100 c.c.; 20 c.c. of iodine were taken, and 5 c.c. of starch solution were added towards the end as indicator. These conditions are also those of the other experiments, except where otherwise stated. Iodine being volatile, it is to be expected that with hot solutions low results will be obtained.

Temperature 15 20 40 60 80 "Hypo" required 20.4 c.c. 20.4 c.c. 20.1 c.c. 19.2 c.c. 15.5 c.c.

These show that the temperature should not much exceed 20.

~Effect of Exposure of the Iodine Solution.~--Twenty c.c. of the iodine were diluted to 100 c.c., and exposed for varying lengths of time in open beakers at the ordinary temperature, and then t.i.trated.

Time exposed -- 1 day 2 days 3 days "Hypo" required 20.4 c.c. 16.1 c.c. 13.6 c.c. 9.4 c.c.

~Effect of Varying Bulk.~--These experiments were carried out in the usual way, bulk only varying.

Bulk 100.0 c.c. 200.0 c.c. 300.0 c.c. 500.0 c.c.

"Hypo" required 20.4 " 20.4 " 20.4 " 20.4 "

~Effect of Varying Acid.~--These experiments were under the usual conditions, the bulk being 100 c.c. The results were--

Acetic acid -- 1.5 c.c. 30.0 c.c.

"Hypo" required 20.4 c.c. 20.7 " 20.7 "

Hydrochloric acid -- 1.5 c.c. 15.0 c.c.

"Hypo" required 20.4 c.c. 20.6 " 20.9 "

Sulphuric acid -- 0.5 c.c. 20.0 c.c.

"Hypo" required 20.4 c.c. 20.7 " 15.2 "[86]

Nitric acid -- 0.5 c.c. 10.0 c.c.

"Hypo" required 20.4 c.c. 21.5 " could not be t.i.trated.

In the application of this t.i.tration to the a.s.say of manganese ores, hydrochloric and hydriodic acids are the only ones likely to be present.

~Effect of Alkalies.~--On theoretical grounds the presence of these is known to be inadmissible. A solution rendered faintly alkaline with ammonia required only 11.2 c.c. of "hypo;" and another, with 0.5 gram of caustic soda, required 4.0 c.c. instead of 20.4 c.c. as in neutral solutions.

~Effect of nearly Neutralising Hydrochloric Acid Solutions with Ammonia.~--Provided care is taken not to add excess of ammonia, this has a good effect, counteracting the interference of excess of acid.

Thus 20 c.c. of iodine (as before) required 20.4 c.c. of "hypo;" with 15 c.c. of hydrochloric acid 20.7 c.c. were required, but with 15 c.c. of acid, nearly neutralised with dilute ammonia 20.4 c.c. were used.

~Effect of the Addition of Starch.~--The addition of varying quant.i.ties of starch has no effect, provided it is added when the t.i.tration is nearly finished, as the following experiments show:--

Starch added 1.0 c.c. 5.0 c.c. 10.0 c.c. 50.0 c.c.