A Text-book of Assaying: For the Use of Those Connected with Mines Part 66

Ash ---- "

contains sulphur ---- per cent.

~Determination of Sulphur.~--The sulphur exists in the coal partly in organic combination, partly as metallic sulphide (iron pyrites, marcasite, &c.), and, perhaps, as sulphate. So that the sulphur determination must be separately reported, since a portion will go off with the volatile matter, and the remainder would be retained and weighed with the c.o.ke.

The sulphur is thus determined:--Take 1 gram of the coal and mix with 1.5 gram of a mixture of 2 parts of calcined magnesia and 1 part of carbonate of soda, and heat in a platinum crucible for one hour or until oxidation is complete. Turn out the ma.s.s and extract it with water and bromine, filter, acidulate with hydrochloric acid, boil off the bromine, and precipitate with baric chloride (estimating gravimetrically as given under _Sulphur_). Another method is as follows:--Take 1 gram of the coal and drop it gradually from a sheet of note paper on to 5 grams of fused nitre contained in a platinum dish. Extract with water, acidify with acetic acid, and estimate volumetrically as described under _Sulphur_.

[Ill.u.s.tration: FIG. 73.]

~Calorific Effect of Coals.~--The heat-giving value of a coal is best expressed in the number of pounds of water, previously heated to the boiling point, which it will convert into steam. This is generally termed its evaporative-power. It may be determined by means of the calorimeter (fig. 73). This consists of a gla.s.s cylinder marked to hold 29.010 grains of water. The instrument consists of a perforated copper stand, provided with a socket and three springs. The socket holds a copper cylinder which is charged with 30 grains of the dried coal mixed with 300 grains of a mixture of 3 parts of pota.s.sium chlorate and 1 part of nitre. The charge is well packed in the cylinder and provided with a small fuse of cotton saturated with nitre. Fill the gla.s.s cylinder to its mark with water and take the temperature with a thermometer marked in degrees Fahrenheit. Ignite the fuse and immediately cover with the outer copper cylinder (extinguisher-fas.h.i.+on), which will be held in its place by the springs. The stop-c.o.c.k should be closed before this is done. Place the apparatus quickly in the cylinder of water. When the action is over open the stop-c.o.c.k and agitate the water by raising and lowering the instrument a few times. Again take the temperature. The rise in temperature, plus 10 per cent. for the heat used in warming the apparatus and lost by radiation, gives the evaporative-power.

The following is an example:--

Temperature before experiment 67.0 F.

Temperature after " 79.0 "

-------- Rise 12.0 "

+ 1/10th 1.2 "

-------- Gives 13.2 "

One pound of the coal will evaporate 13.2 pounds of water.

SHALES, ETC.

The a.s.say of these is carried out in the same way as that of coals, but the volatile matters are separately examined, and, in consequence, a larger quant.i.ty of material must be used. For the moisture, volatile matter, fixed carbon and ash, the determinations are the same, but a special distillation must be made to obtain a sufficient quant.i.ty of the volatile products for subsequent examination. Take 500 or 1000 grams of the well-sampled and powdered shale, and introduce into a cast-iron retort as shown in fig. 74. Lute the joint with fire-clay, place the cover on, and bolt it down. The bolts should have a covering of fire-clay to protect them from the action of the fire. Place the retort in a wind furnace, supporting it on a brick, and pack well around with c.o.ke. Build up the furnace around and over the retort with loose fire-bricks, and heat gradually.

[Ill.u.s.tration: FIG. 74.]

As soon as water begins to drip, the tube of the retort is cooled by wrapping a wet cloth around it, and keeping wet with water. The water is kept from running into the receiver by a ring of damp fire-clay. A quant.i.ty of gas first comes over and will be lost, afterwards water and oily matters. The retort must be red hot at the close of the distillation, and when nothing more distils off, which occurs in about two or three hours, the wet cloth is removed, and the tube heated with a Bunsen burner to drive forward the matter condensed in it into the receiver, and thus to clean the tube. It can be seen when the tube is clean by looking up through it into the red-hot retort. The receiver is then removed, and the retort, taken from the furnace, is allowed to cool. When cold it is opened, and the fixed carbon and ash weighed, as a check on the smaller a.s.say.

The distillate of water and oil is warmed, and will separate into two layers, the upper one of which is oil, and the lower water. These are measured, and if the specific gravity of the oil is taken, its weight may be calculated. If the two liquids do not separate well, the water may be filtered off, after cooling, through a damped filter. The separation is, however, best effected in a separator (fig. 75). The liquids are poured into this, allowed to settle, and the lower layer drained off. The volume of the water is measured and its weight calculated in per cents. on the amount of shale taken.

[Ill.u.s.tration: FIG. 75.]

~Examination of the Oil.~--A sufficient quant.i.ty of the oil must be got, so that if one distillation does not yield enough, the requisite quant.i.ty must be obtained by making two or more distillations. The oils are mixed, and the mixture, after having had its volume and specific gravity ascertained, is placed in a copper retort, and re-distilled with the aid of a current of steam. The residue in the retort is c.o.ke.

The distillate is separated from the water by means of the separator, and shaken for ten minutes with one-twentieth of its bulk of sulphuric acid (sp. g. 1.70). The temperature should not be allowed to rise above 40. Allow to stand, and run off the "acid tar."

The oil is now shaken up with from 10 c.c. to 20 c.c. of sodic hydrate solution (sp. g. 1.3), allowed to stand, warmed for half-an-hour, and the "soda-tar" run off.

On mixing this soda-tar with dilute acid, the "crude shale oil creosote"

separates, and is measured off.

The purified oil is next re-distilled in fractions, which come over in the following order:--"Naphtha," "light oil," "heavy oil," and "still bottoms." For the first product, which is only got from certain shales, the receiver is changed when the distillate has a specific gravity of 0.78. For the second product the process is continued till a drop of the distillate, caught as it falls from the neck of the retort on a cold spatula, shows signs of solidifying. This is "crude light oil."

The receiver is changed, and the "heavy oil" comes over; towards the end a thick brown or yellow viscid product is got. The receiver is again changed, and the distillation carried to dryness.

The "crude light oil" is washed cold with 2 per cent. of sulphuric acid (concentrated), and afterwards with excess of soda. Thus purified it is again distilled to dryness, three fractions being collected as before.

Naphtha, which is added to the main portion, and measured; "light oil,"

which is also measured; and "heavy oil," which is added to that got in the first distillation. This last is poured into a flat-bottom capsule, and allowed to cool slowly. The temperature may with advantage be carried below freezing-point. The cooled cake is pressed between folds of linen, and the paraffin scale detached and weighed.

The results may be reported thus:--

Naphtha, sp. g. ---- Light oil, sp. g. ---- Heavy oil, sp. g. ---- Paraffin scale ---- c.o.ke, &c. ----

The results are calculated in per cents. on the oil taken. Some workers take their fractions at each rise of 50 C. The composition of average shale, as given by Mills, is as follows:--Specific gravity, 1.877; moisture, 2.54.

Gas } Volatile matter, water, ammonia } 23.53 Oil } Fixed carbon 12.69 Ash 63.74 _____ 99.96

The ash is made up of silica, 55.6; ferric oxide, 12.2; alumina, 22.14; lime, 1.5; sulphur, 0.9; soluble salts (containing 0.92 per cent.

sulphuric oxide), 8.3.

Total sulphur in shale 1.8 per cent.

" " in ash 1.3 "

For further information on these a.s.says, and for the a.s.say of petroleums, bitumens, &c., the student is referred to Allen's "Commercial Organic a.n.a.lysis," Vol. II.

~Determination of Organic Carbon in a Limestone.~--Take 1 or 2 grams and dissolve with a very slight excess of dilute hydrochloric acid, evaporate to dryness, and determine the carbon in the residue by combustion with copper oxide.

~Estimation of Carbon in a Sample of Graphite (Black-lead).~--Weigh up 1 or 2 grams in a dish and calcine in the m.u.f.fle till the carbon is burnt off. Weigh the residue, and calculate the carbon by difference.

[Ill.u.s.tration: FIG. 76.]

[Ill.u.s.tration: FIG. 77.]

[Ill.u.s.tration: FIG. 78.]

~Determination of Carbon in Iron.~--The carbon exists in two states--free (graphite) and combined. The following process estimates the total carbon:--The carbon existing as graphite may be separately estimated in another portion by the same process, but using hydrochloric acid to dissolve the iron instead of the copper solution:--Weigh up 2 grams of the iron (or a larger quant.i.ty if very poor in carbon), and attack it with 30 grams of ammonic-cupric chloride[119] dissolved in 100 c.c. of water. Let the reaction proceed for a quarter-of-an-hour, and then warm until the copper is dissolved. Allow to settle, and filter through a filtering-tube. This is a piece of combustion tube drawn out and narrowed at one end, as shown in fig. 76. The narrow part is blocked with a pea of baked clay, and on this is placed half-an-inch of silica sand (previously calcined to remove organic matter), then a small plug of asbestos, and then a quarter-of-an-inch of sand. The tube is connected with a pump working at a gentle pressure, and the solution is filtered through the tube with the aid of a small funnel (fig. 77). The residue is washed, first with dilute hydrochloric acid, and then with distilled water. The tube is dried by aspirating air through it, and gently warming with a Bunsen burner. The tube is then placed in a small combustion-furnace, and connected with calcium chloride and potash bulbs, as shown in fig. 78. The potash bulb to the right of the figure must be weighed. A slow stream of air is drawn through the apparatus, and the heat gradually raised; in from thirty minutes to one hour the combustion will be complete. The potash bulbs are then disconnected and weighed, and the increase multiplied by 0.2727 gives the weight of carbon.

CARBONATES.

Carbon dioxide, which is formed by the complete oxidation of carbon, is a gas with a sweetish odour and taste, having a strong affinity for alkalies, and forming a series of compounds termed carbonates. The gas itself occurs in nature, and is sometimes met with in quant.i.ty in mining. The carbonates occur largely in nature, forming mountain ma.s.ses of limestone, &c. Carbonates of many of the metals, such as carbonate of lead (cerussite), carbonate of iron (chalybite), carbonates of copper (malachite and chessylite), and carbonate of magnesia (magnesite), are common.

All the carbonates (those of the alkalies and alkaline earths excepted) are completely decomposed on ignition into the oxide of the metal and carbon dioxide; but the temperature required for this decomposition varies with the nature of the base. All carbonates are soluble with effervescence in dilute acids; some, such as chalybite and magnesite, require the aid of heat. The alkaline carbonates are soluble in water; the rest, with the exception of the bicarbonates, are insoluble therein.

Carbonates are recognised by their effervescence with acids--a stream of bubbles of gas are given off which collect in the tube, and possess the property of extinguis.h.i.+ng a lighted match. The most characteristic test for the gas is a white precipitate, which is produced by pa.s.sing it into lime or baryta-water, or into a solution of subacetate of lead.

The expulsion of carbon dioxide by the stronger acids serves for the separation of this body from the other acids and bases.

~Dry a.s.say.~--There is no dry a.s.say in use. Any method which may be adopted will necessarily be applicable only to special compounds.

WET METHODS.

There are several methods in use which leave little to be desired either in speed or accuracy. We will give (1) a gravimetric method in which the estimation may be made directly by weighing the carbonic acid, or, indirectly, by estimating the carbon dioxide from the loss; (2) a volumetric one, by which an indirect determination is made of the gas; and (3) a gasometric method, in which the volume of carbon dioxide given off is measured, and its weight deducted.

[Ill.u.s.tration: FIG. 79.]

[Ill.u.s.tration: FIG. 80.]