until crude petroleum was produced in that country in such
abundance, and at so low a cost, that the distillation of bituminous
minerals became unprofitable. The highly bituminous
Boghead coal, or Torbanehill mineral, which yielded 120 to 130
gallons of crude oil per ton, was worked out in 1862, and since
then the Scottish mineral oils and paraffin have been obtained
from the bituminous shales of the coal-measures, the amount of
such shale raised in Great Britain in 1907 being 2,690,028 tons.
The following list represents an attempt to assign a geological age to the various occurrences of oil-shale and similar substances throughout the world:—
| Oil-Shales | |
| Geological System. | Locality. |
| Miocene | France (Vagnas), Servia. |
| Eocene | Brazil. |
| Cretaceous | Syria, Montana, New Zealand. |
| Neocomian | Spain. |
| Jurassic | Dorset, Württemberg. |
| Permian | France (Autun, &c.). |
| Carboniferous | Scotland, Yorkshire, Stafford, Flint, France, Nova Scotia. |
| Kerosene-Shale | |
| Permo-Carboniferous | Queensland, New South Wales, Tasmania. |
| Tar-Lignite | |
| Miocene | Moravia, Lower Austria, Bavaria. Rhenish Prussia, Hesse, Saxony. |
| Oligocene | Bohemia, Tirol. |
Oil-Shale.—The oil-shale of Scotland is dark grey or black, and has a laminated or horny fracture. Its specific gravity is about 1·75, and 20 cub. ft. of it weigh rather less than a ton. The richer kinds yield about 30 gallons of oil per ton of shale, and in some cases as much as 40 gallons, but the higher yield is usually obtained at the expense of the solid paraffin and of the quality of the heavy oils. The inferior shales yield about 18 gallons of oil, but a much larger amount of sulphate of ammonia. The oil consists chiefly of members of the paraffin and olefine series, and thus differs essentially from that obtained from true coal-shales, in which the hydrocarbons of the benzene group are largely represented.
A full account of the Scotch shale-oil industry, as the most important and typical, will be given later, the corresponding industries in other countries and districts being dealt with first.
In addition to the Carboniferous oil-shales of Flint and Stafford, the Kimmeridge shale, a bluish-grey slaty clay, containing thin beds of highly bituminous shale, occurs in Dorsetshire, and has from time to time attracted attention as a possible source of shale-oil products. The so-called “Kerosene-shale” of New South Wales has been extensively mined, and the industry is now being developed by the Commonwealth Oil Corporation, Ltd. The French shale-oil industry is much older than that of Scotland, but has made far less progress, the amount of shale distilled in 1897 being 200,000 tons, as compared with 2,259,000 tons in Scotland. The shales of New Zealand have never been extensively worked, the production having decreased instead of increased. Oil-shale of good quality occurs in Servia, and has been found to yield from 4312 to 5412 gallons of oil per ton. The production of mineral oils and paraffin by the distillation of lignite is carried on in Saxony, the mineral worked being a peculiar earthy lignite, occurring within a small portion of the Saxon-Thuringian brown-coal formation. Other occurrences of this mineral have been indicated in the list of localities above.
The Shale-Oil Industry of Scotland.—The modern development of the shale-oil industry of Scotland dates from the commencement of Robert Bell’s works at Broxburn in 1862.
The oil-shales are found in the Calciferous Sandstone series, lying between the Carboniferous Limestone and the Old Red Sandstone. They occur at several points in the belt of Carboniferous rocks across the centre of Scotland, for the most part in small synclinal basins, the largest of which is that at Pentland, where the levels are 2 m. long, without important faults. Mining is carried on, where the seams are over 4 ft. thick, by the “pillar and stall” system; seams under 4 ft. are worked by the “longwall” system. The shale is blasted down by gunpowder, and passed over a 1-in. riddle, the smalls being left underground. Before being retorted the shale is passed through a toothed breaker, which reduces it to flat pieces 6 in. square. These fall into a shoot, and thence into iron tubs of 10 to 25 cwt. capacity, which run on rails to the tops of the retorts.
The retorts in which the shale is distilled have undergone considerable variation and improvement since the foundation of the industry. Originally horizontal retorts, like those used in the manufacture of coal-gas, were employed, and the heavy oils and paraffin were burned as fuel. When the latter product became valuable vertical retorts were adopted, as the solid hydrocarbons undergo less dissociation under these conditions. Steam was employed to carry the oil vapours from the retort. The earliest form of vertical retort was circular (2 ft. in diameter) or oval (2 ft. by 1 ft. 4 in.) and 8 or 10 ft. long. Six or eight of these were grouped together, and the heating was so effected that the bottoms of the retorts were at the highest temperature. They were charged by means of hoppers at the top, the exhausted shale being withdrawn through a water-seal every hour and fresh added, whence this is known as the “continuous system.”
In the first Henderson retort (1873) the spent shale was used as fuel. The retorts, which were oblong in cross-section, were arranged in groups of four, and had a capacity of 18 cwt. They were charged in rotation, as follows: when a sufficient temperature had been attained in the chamber containing them, one retort was charged from the top, and in four hours the one diagonally opposite to it was charged. After eight hours the one next to the first was charged, and after twelve hours the fourth. Up to the sixteenth hour only ordinary fuel was used in the furnace, but the spent shale from the first retort was then discharged into it. The other retorts were similarly discharged in the above order at intervals of four hours, each being at once recharged. The shale was black when discharged, but soon glowed brightly. Owing to the small amount of carbon in the spent shale, only a slow draught was kept up. The outlet for the oil vapours was at the lower and less heated end of the retorts, and steam, which had been superheated by passage through pipes arranged along one side of the retort chamber, was blown in copiously through pipes to aid in the uniform heating of the shale and to continuously remove the oil vapours, dissociation from overheating being thus minimized. It was believed that a temperature of about 800° F. produced the best results. This retort was worked on what is known as the “intermittent system.”
The Pentland Composite retort (1882) and the later Henderson type (1889) were both continuous-working and gas-heated, the second being a modification of the first, designed with a view to obtaining a larger yield of sulphate of ammonia without detriment to the crude oil. In both the upper part of the retort was of cast iron and the lower of fire-clay. The upper portion was heated to a temperature of about 900° F. whilst the lower was maintained at about 1300° F. The charge in the retort gradually travelled down, owing to the periodical removal of spent shale at the bottom, and the descent was so regulated that no shale passed into the highly-heated part until it had parted with the oil it was capable of yielding. The shale, however, still contained nitrogen, which in the presence of steam produced ammonia at the higher temperature.
The three classes of retorts now employed in the distillation of shale in the Scottish oil-works are covered by the following patents:—
1. In use at Pumpherston, Dalmeny and Oakbank—No. 8371 of 1894; No. 7113 of 1895; No. 4249 of 1897.
2. In use by Young’s Paraffin Light and Mineral Oil Company, Ltd.—No. 13,665 of 1897; No. 15,238 of 1899.
3. In use by the Broxburn Oil Company, Ltd.—No. 26,647 of 1901. The objects of the invention for which patent No. 8371 of 1894 was granted to Bryson (of Pumpherston Oil Works), Jones (of Dalmeny Oil Works), and Fraser (of Pumpherston Oil Company, Ltd.), are described in the specification as “to so construct the retorts and provide them with means whereby fluxing or dandering of the substance being heated is prevented in the retorts: also to effect an intermittent, continuous, or nearly so, movement within the retort.” In order to carry out these objects, the bottom of the
