Petroleum

by Albert Lidgett

The usual dimensions of the inclined shaft are a width of from 10 to 12 feet, and the height is from 6 to 8 feet. If the sides of the shaft prove to be of a soft nature, as is generally the case with the shale at the crop, walls are run up and the roof is supported by larch crowns, but, where the shale is hard and the roof good, then the less costly method of timbering is adopted.

The supports to the roof in many cases are fixed “centre” fashion, dividing the shaft into two unequal parts. The smaller division has generally a width of just over 3 feet, and is used for haulage ropes and water pipes, while the larger division is utilized for winding. During the progress of sinking, levels are broken away in the seam at regular distances, and driven so as to get communication with, and drive headings to form, the outer mine. These headings are driven in the same direction as the sinking mine to the levels above, until they connect with the outer mine or shaft. The outer mine is then used for winding the shale up to the surface, and the other is kept for sinking purposes, and by this means winding and sinking can go on simultaneously.

GENERAL VIEW OF THE PUMPHERSTON WORKS

The seams of shale in the Midlothian fields vary generally from 4 to 10 feet in thickness, say 7 feet as an average, and, on the whole, they are comparatively free from ribs of unproductive rock. With a thickness of 7 feet, experience has shown that the method best adapted for the efficient working of the shale is “stoop and room,” but in the case of two seams of shale, separated by a bed of foreign material of sufficient thickness for packing, the long wall method proves the more suitable. The “stoop and room” method, however, is more generally used throughout the Scottish shale district than any other, its chief characteristics being the (1) “whole” or first working, and (2) the broken or second working. The whole working consists of a series of excavations made in the shale, whereby it is divided into rectangular blocks or pillars. These excavations are called rooms, one set being driven at right angles to the dip of the shale and at regular distances from one another, and commonly called “levels”; another set, driven to the rise of these levels and at right angles to them, being usually known as “ends” or “upsets.” The latter are broken off the levels at regular intervals and driven upwards to meet the levels above.

The shale miner holes as far as he can reach—probably three or more feet—and brings down the shale by blasting, the process being repeated until he penetrates a distance of from 9 to 12 feet from the face at road-head. The shale, being loosened from its natural bed, is then placed in “hutches,” which are taken to the bottom of the shaft by either horse or chain haulage (much as with coal), and then the journey to the mouth is commenced. Before leaving the question of shale mining, it should be explained that the shale miner is subject to dangers much as his colleague in the coal-pit, but the volume of gases found in the shale seams is not so great as in the coal measures. These, however, are of an explosive nature, the most common being fire-damp.

Once above ground, the shale is conveyed to breaking machines by endless wire-rope haulage. Passing through the machines, it is broken into suitable sizes for distillation, and drops into hopper-shaped hutches. These hutches have a capacity of about a ton, and each in turn is conveyed to the top of the retorts on an inclined scaffold by an endless chain. The shale then falls by the operation of a lever into a hopper or magazine communicating directly with the retorts, one hopper with a storage capacity of 24 hours’ supply of shale being connected to each retort of the Pumpherston Company.

This Company’s retorts—they are patented—are in use at the various works of the Pumpherston Company, and are an interesting feature to visitors. The shale is fed by gravitation into cylindrical-shaped retorts, and built vertically in ovens of four, each oven having four chambers. The upper portion is of cast-iron, 11 feet long by 2 feet in diameter at the top, and slightly enlarged toward the bottom. Heat is applied externally from the incondensable gases obtained from the distillation of the shale, and this heat is made to circulate round the retort. In the case of the poorer qualities of the shale, however, the heat is assisted by producer-gas. The heating gas enters near the bottom portion of the retort, which is of fire-brick, along with a certain quantity of air, and a high temperature—from 1,200°F. to 1,600°F.—is maintained, in this portion converting the nitrogen of the shale into ammonia, which is preserved by a continuous supply of steam delivered at a slight pressure at the bottom of the hopper.

The oil gases are distilled from the shale in the cast-iron portion of the retort at a temperature of about 900° F., and, along with the ammonia gas, are drawn off by the exhausters through a branch pipe at the top of the retort, through the atmospheric condensers, from which the condensed liquid oil and water containing ammonia flow into a small separator tank. It is here that, owing to their different specific gravities—for one is lighter than the other—they assume different levels, and are thus drawn off into separate tanks. The gases then pass through ammonia scrubbers, in which they are washed for ammonia, and then through the naphtha scrubbers, where the lighter gases, which could not be caught in the atmospheric condensers, are washed with oil and a good quality of light oil or naphtha is recovered. The incondensable portion passing from these scrubbers is burned in the retorts as previously mentioned. With a shale of average yield, the retort can be heated by these incondensable gases from the distillation, and a surplus obtained for burning under steam boilers.

What is doubtless a very unique feature of the Pumpherston retort is the mechanical arrangement for withdrawing the spent shale continuously, and thus keeping the whole mass inside the retorts in constant movement. Below each pair of retorts is fixed a hopper made of cast-iron, and fixed to girders supported on the brick piers or columns between the ovens. At the top of each hopper, and immediately underneath the bottom of the retorts, is fixed a cast-iron disc or table, with a space left between its edge and the sides of the hopper. The whole mass of shale in the retort rests upon the table, the space permitting some to pass over the edge. Through the centre of the table a steel spindle projects, on the upper end of which is fixed a curved arm, and this, when rotated, pushes some of the shale off, causing it to fall over the edge of the table into the hopper below. The shaft carrying the curved arm passes through a stuffing-box on the hopper, and has a ratchet and lever fitted to the lower end, actuated by a rod of T-iron which is made to travel horizontally, and is driven by a small electric motor. The motion is comparatively slow, the arm making but one revolution in about 20 minutes, but the action is most satisfactory, the through-put of shale being regulated at will.