GAS-MAKING AND OTHER COALS
367
different velocities. A particle of given dimensions and specific gravity when falling through a liquid attains a maximum limiting velocity, which is dependent upon the specific gravity and diameter of the particle. If, therefore, we have two particles of equivalent size, it follows that their specific gravities must diller in order that separation may be efiected. At the same time, velocity depends upon diameter, and it is important to note that if, for example, we had a particle of coal (1-3 sp. gr.) and a particle of shale (2-3 sp. gr.), the diameter of the coal being 4-3 times that of the shale, then both. would fall at the same rate. To overcome this difficulty, therefore, it is customary in most cases to pass the coal firstly through a 2-in.ch. screen, the screened material being waslied, while the larger coal is dealt with on a hand-picking belt.
For an average mixture of coal and shale the velocity may be calculated from the following formula (Eittinger):—
V= 1-28
where V = velocity in feet per second.
D = average diameter of particles in feet.
d = specific gravity of the material.
Knox has prepared a most interesting graph (Fig. 256) which illustrates the limits within which separation of different sizes of particles of various specific gravities can be efiected. If any horizontal line of the graph is taken it gives the relative sizes of the particles which fall with equivalent velocities. For example, a particle of pyrites inch diameter, one of shale with pyrites J inch diameter, one of shale | inch diameter, and one of coal 1| inches diameter, would all fall, or be floated, at the same velocity. It will be appreciated, therefore, that to ensnre efficient separation
it is essential that the sizing should be such that
heavy and light particles do not assume an equivalent velocity. In other, words the sizing must be such that the rate of fall of the largest light particle is rather less
than the velocity of the smallest lieavy particle.
Coal-Washing Machineby
The principles involved in the plant employed for separating impurities from coal may briefly be summarized as follows:—