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Colliery Engineering

August, 1951

Chlorine

Chlorine is present in British coals from 0.01 to 1 per cent. Its usual form is as alkali chlorides, mainly sodium chloride. There is some evidence that it is distributed throughout the coal. It source may be the original plants and infiltered water. Coals from the areas of South Yorkshire, North Derbyshire and the Stoke district of Staffordshire are high in chlorine, whereas Welsh coals, for example, are low. The chlorine in coal cannot wholly be extracted by water. Cobb¹⁶ showed that 100 g. of coal, from which 0.38 g. of chlorine could be extracted as chloride in four hours by hot water, yielded a further 0.41 g. on extraction with 10 per cent. nitric acid. The effect of the high chlorine content of coals is harmful. In high-pressure boilers where the temperature of combustion is high, the alkali chlorides are volatilised ; they then react with sulphur dioxide and sulphur trioxide to form bonded deposits on the boiler tubes. In fact, it is possible, as Crossley has shown, to classify coals according to their chlorine content as a measure of the extent of deposit. In a survey of 28 power stations using mechanical stokers with coals of chlorine content greater than 0.15 per cent., they all reported the main cause of stoppage to be deposits on the front bank of tubes and the superheater. The average time the boiler was in operation was 1,000 hours. Power stations using similar coals, but in the pulverised form, had no equivalent deposits. The reason for their immunity is the presence of an excess of ash particles which prevent the formation of a bonded deposit. There is also a shorter time in a pulverised-fuel boiler for volatilisation of the alkali salts and reaction with the sulphur oxides.

In certain cases there is evidence of corrosion by hydrochloric acid, particularly when burning cokes rich in chlorine. This was noticeable in the coolers and filtering medium of small mobile producers, used during the war as substitutes on petrol-driven engines.

Chlorine in coal can influence the volatility of other elements present in the coal. One example is arsenic. A coal containing 60 parts of arsenic as As₂O₃ per million, 0.02 per cent. of chlorine, and an unusually high proportion of carbonate when burnt, retained practically all the arsenic in the ash, but adding chloride to increase the chlorine content to 1 per cent., reduced the arsenic in the ash by over a half.

During the high-temperature carbonisation of coal, chlorides (Cl > 0.05 per cent.) can have a very deleterious effect on the refractory walls of the retorts or ovens. The salt penetrates the brickwork, and a point is reached where the temperature is sufficiently high to cause fusion of the compound formed of sodium, alumina, and silica. The brick then cracks and pieces break away. There may also be volatilisation of iron in the fireclay as chloride. It is one of the reasons why silica has displaced firebrick in modern retorts and ovens.

Fluorine

The effects of fluorine have only recently been observed. Lessing¹⁷ observed in 1934 the breakdown of porcelain ring packing in a concentrated-ammonia-liquor scrubber at the Bristol gasworks. This was attributed to fluorine compounds in the liquor being distilled. Crossley¹⁸ about the same time showed that the etching of glassware in an annealing kiln was due to fluorine compounds from the combustion of the coal used. Methods for determining fluorine in coal were developed by Crossley, and in both cases the coals were found to contain 85 to 140 parts per million of fluorine. A survey of British coals (see Table I) showed that the fluorine content varied from 5 to 200 parts per million. The fluorine in coal has been shown to be in the form of fluorapatite, so that the phosphorus content of coal gives an indication of the fluorine content. (See Table II.) On combustion at high temperature the fluorine is evolved, probably as hydrogen fluoride, silicon fluoride, or as boron fluoride. These compounds may take part in deposit formation in association with phosphoric acid and may cause corrosion. Coals used for the drying of malt by direct contact with the gases are chosen with low arsenic content to avoid contamination of the malt and subsequent beer. Similarly, the danger of contamination with fluorine is possible with high-fluorine coals. In a direct contact kiln the volatile fluorine compounds formed on combustion are absorbed by the malt.

Arsenic

Arsenic is associated with the mineral matter of coal, and occurs in conjunction with pyrites as mispickel FeS₂.FeAs₂. The amount in coal varies from 1 to about 75 parts per million, expressed as arsenious oxide (As₂O₃). On combustion the arsenic is volatilised as oxide, unless retained as arsenite and arsenate by the carbonate minerals. As mentioned earlier, chlorine in the coal can increase the proportion of arsenic volatilised.

Trace elements

There are a number of elements which are closely associated with the coal substance. This is shown by the fact that when the adventitious matter is removed by a coal-cleaning process, the proportion of these elements in the ash of the cleaned coal increases. The pioneer work in this field was undertaken by the late Dr. Goldschmidt¹⁹, who endeavoured to correlate the elements in the soil with the ash of plants, and that of humus and fossil fuels. He showed there was an enrichment of certain elements taking place. Some of these trace elements are necessary for the life of the plant. Examples are copper, cobalt, zinc and boron. It has been shown also that some of these elements are necessary for man, and are absorbed in his natural food.

More than thirty elements have been found to be associated with the organic matter of coal. Examples are boron, germanium, gallium, and vanadium.

Boron is found associated with both the mineral matter and the organic matter, and in this resembles sulphur. The amount varies from about 5 to 130 parts per million. The boron in the mineral matter is found in shale bands, and in the shale of the roof and floor of the coal mine. It may also be found as fine particles, suggesting that it is waterborne and deposited during coal formation. Small particles of tourmaline (a boro-silicate) have been recorded in American coals.

On combustion of coal the boron is volatilised, and deposits on boiler tubes have been found with a borate content as high as 15 per cent. B₂O₃. This is due to fractional condensation of the borate in the boiler system.

During carbonsiation some boron has been found in the tar and ammonia liquor, but the type of boron compounds in the tar is not known.

Lead and zinc have been found in many coals. The highest zinc found was 0.7 per cent. Many of these trace elements when coal is carbonised tend to concentrate in the tar. Green and Thakur²⁰ found a zinc-pyridine complex in the soluble part of the free carbon. As much as 0.04 per cent. zinc was found in the tar, and as much as 1-2 per cent. in the free carbon. Other elements found were lead, silicon, iron, cobalt, manganese and strontium. It is not known whether these elements are present as inorganic constituents or as organic complexes.

Among the rare elements which have aroused interest in recent years is germanium, the supplies of which are limited. The detection of this in any quantity in coal ash was first reported by Goldschmidt¹⁹. He was examining spectrographically a number of German coals for trace elements, and as a comparison obtained from his local museum collection at Gottingen a 70-year-old sample of English coal labelled "Hartley, near Newcastle." The ash from this sample contained the phenomenally high figure of 1.1 per cent. of germanium. The coal came from the Hartley Main Collieries Co. Ltd., which at the period the sample was taken was working the Low Main seam. It proved to be the richest specimen in germanium which has ever been examined. One other picked sample from the Yard seam of the same colliery showed 1 per cent, of germanium in the ash, but an examination of over a hundred samples from the same and adjacent seams showed no figures higher than 0.5 per cent. in the coal ash. The two samples with the exceptionally high figures were concluded to be from special vitrain, probably "cauldron" vitrain, and not truly representative of the whole seam. There is no doubt that certain areas of Northumberland and Durham are richer in germanium than other coalfields. The highest yields of germanium occur in the bright bands of the coal, especially the vitrains of low ash content. The ash of the vitrain of high germanium content was only 0.9 per cent. Tests of the roof and floor showed no appreciable amount of germanium, showing that the germanium is associated with the inherent mineral matter.

A number of tests on boiler deposits shows the presence of germanium, and figures of 0.2 to 0.9 per cent. of germanium have been reported. Consideration has been given to the possible recovery of germanium from selected dusts. Germanium is volatilised from coal during combustion, particularly in a reducing atmosphere, and its volatility is increased if chlorides are present. It is this volatility which is responsible for the finding of germanium in the flue dust, rather than in the residual ash of the coal. In an address at a recent meeting of the General Electric Company, the chairman, Sir Harry Railing, referred to the search made in 1946 for supplies of germanium for use in modern crystal diodes and triodes. A source was found in the flue dusts from Northumberland and Durham coals, and processes for the extraction of germanium in a high state of purity had been developed.

When coal is carbonised, some germanium is found in the tar and is associated with the mineral matter of tar.

Another element which behaves in a similar manner to germanium is gallium. Again, this is found in larger quantities in the Northumberland and Durham coalfields. Flue dusts have been found derived from coals in these fields with from 0.04 to 0.55 per cent. of gallium. One exceptional sample contained 1.58 per cent.

Vanadium, which is present in the ash of petroleum, also occurs in coal ash, but to a very small extent up to 0.03 per cent. V0₅. It appears to be associated with the coal substance. High figures have been found for "cauldron" vitrains, and in vitrain-like bands embedded in the carboniferous shales and sandstones in the Peacock seam, North Staffordshire²². "Cauldron" vitrain is a type of vitrain associated with large conical masses of shale ("cauldrons"), occurring in the measures overlying coal seams in certain areas, and apparently formed from the lower parts of trees Jones and Miller²¹ reported a number in the shales overlying the Yard seam in Northumberland, and the Busty and Brockwell seams of Durham.

The high figures for vanadium, titanium, chromium and nickel in the accompanying table should be noted. Figures for North Wales vitrains (floats) show up to 6.9 per cent. vanadium oxide in the ash, and for cauldron vitrain (floats) from the Brockwell seam, Durham, 11.3 per cent. vanadium oxide. These are exceptional figures for special materials, and should not be taken as typical of the coal seams themselves. It is of interest that Whitby jet similarly gives a high figure of 11.6 per cent. vanadium oxide in the ash, which is only 0.4 per cent. of the jet.

This association of these uncommon elements with the organic material of the coal has been the subject of much speculation. Goldschmidt¹⁹, to whose work reference has been made previously, considered the origin of these elements to be the soil. The plant absorbs the solutions, the amount of substance absorbed being proportional to its solubility. The salts are concentrated in the leaves, and when these fall to the ground there is an enrichment of the humus. Analyses have shown that there is a higher percentage of these "trace" elements in the leaves and humus than in the subsoil. These "trace" elements may be precipitated as insoluble salts, or may form organometallic complexes with the coal substance.

Other elements which are of importance are barium, lead and copper. These are associated with the mineral matter of coal.

Barium occurs as witherite — barium carbonate — and Briggs²³ in 1902 reported a vein of barium carbonate and sulphate in the coal at Brancepeth colliery, varying from a few inches to 16 ft.

Another colliery in the neighbourhood found a thick vein of witherite, and supplied a considerable proportion of the world’s requirements. These veins spread into the coal, and are responsible for the barium oxide found in coal ash.

Lead and copper are found associated with the pyrites of the coal, as also is zinc. As much as 2 per cent. of copper oxide has been reported in the ash from a Midland coal. Dunn and Bloxham²⁴ examined the pyrites from several coals, and found lead to the extent of 1 to 461 parts per million, and copper from a trace to 80 parts per million. They showed the presence of lead in dusts and on grass. The content in herbage was in sufficient amounts in certain instances to be toxic.

For further information on trace elements, such publications should be consulted as "Rare and Uncommon Chemical Elements in Coal" by F. H. Gibson and W. A. Selvig²⁵, Tech. Paper 669, U.S. Bureau of Mines; and the "Inorganic Constituents of Coal" by H. E. Crossley²⁶.

Summarising the elements considered in this article, their association with the coal substance and their proportions in British coals are as follows :—

The author’s thanks are due to Dr. Horton and Mr. Aubrey, of the Fuel Research Station, for their assistance in the preparation of this article, which is published by permission of the Director of Fuel Research, Department of Scientific and Industrial Research. The illustrations are Crown copyright, and are published by permission of the Controller, H.M. Stationery Office.

REFERENCES

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25. Gibson and Selvig, U.S. Bureau of Mines, 1944, Tech. Paper 669.
26. Crossley, Inst. of Fuel Wartime Bulletin, 1946, 57.

Drawings and Photographs accompanying the article