On the Problem of Coal Beneficiation in a Dense Medium¹

I. V. Gebler

At the present time there is a growing interest in the problem of coal beneficiation in suspensions, in connection with the well-known advantages possessed by this method; these include the accuracy of separating fractions in accordance with a specified specific gravity and, in connection with this, the possibility of obtaining high-quality concentrates in terms of ash and sulfur content, which is especially important for coals intended for coking; at the same time, an improvement is achieved in the quality of the middling product with the least losses of coal in the refuse rock.

Coal beneficiation in dense suspensions can advantageously be carried out in combined flowsheets by beneficiating the middling product, thereby replacing its multiple recleanings on jigging machines.

Among the various methods of coal beneficiation in a dense suspension, the most successful is beneficiation in horizontal streams of magnetite suspension with a velocity of the latter not exceeding 0.10–0.15 m/sec, so that the motion of the streams is laminar in character.

The machines belonging to this group, according to literature data [1], with dimensions: length 6200 mm, width 2000 mm, and height 5310 mm, when beneficiating easily washable coal of size 30–6 mm, have a capacity on the order of 60 t per hour.

In general, however, the capacity of a machine of given dimensions depends on the amount of middling fractions discharged per unit time, i.e., on the velocity of motion of the intermediate streams of the suspension.

Therefore, when the yield of middlings is 25–30% for difficult-to-wash coals, the capacity falls sharply and, even at a stream velocity of 0.2 m/sec, amounts to no more than 8 t per hour per 1 m of width.

For coal beneficiation in a dense suspension, the separator described in the present article may be used; its operation was studied by the author on a laboratory model. In its operating principle, with respect to the movement of the material, this apparatus is analogous to a tubular conveyor.

The design of the separator is shown in the attached schematic drawing (Fig. 1) and photograph (Figs. 2 and 3)². The separator consists of a drum 1, open at the sides, made of sheet iron with round (or slot-shaped) openings, or of mesh. The drum rotates in trough 2. On the inner surface of the drum there is a helical rib 3. The drum is located in a dense liquid, which fills the trough to level 001. Coal enters the drum through loading funnel 4. Po-

Separator for coal beneficiation in a heavy liquid

Annotations visible in the drawing: “Along MN”; “View in the direction of arrow D, section along CD”; “View in the direction of arrow V.”

Fig. 1

Fig. 2

the rock sinks and is moved by the screw rib down the drum to its opposite end. Here it enters annular trough 5, in which there are 12 partitions (in the drawing, “blades”) 6; by these partitions the annular trough is divided into 12 compartments.

The sunken fraction, entering these compartments, is lifted as the drum rotates and then falls onto grate 7, from which it goes to the washing screen. The floated coal is moved by the screw rib likewise to the opposite end of the drum, to pocket 8, and is discharged from it by means of an elevator with perforated buckets.

Fig. 3

The apparatus is intended for operation in a suspension of magnetite, but may also be suitable for other suspensions.

To maintain the constancy of the suspension, the drum is provided with blades 10, by means of which continuous mixing of the suspension takes place.

The washing of the coal and concentration of the suspension are carried out in the usual way.

Experiments with the model established that the peripheral speed of rotation of the drum, for maintaining magnetite grains in suspension, should be on the order of \(V = 0.10\ \text{m/sec}\) when the magnetite grain size is \(0.1\ \text{mm}\).

The production apparatus is assumed to have the following dimensions.

Perforated drum: diameter \((D) = 3\ \text{m}\); length \((L) = 4.5\ \text{m}\).

To create a peripheral speed of \(0.10\ \text{m/sec}\), the required number of revolutions will be:

\[ n = \frac{30 \cdot 0.1}{3.14 \cdot 1.5} = 0.65, \]

and \(n = 0.5\ \text{rev/min}\) is adopted.

The screw rib is welded to the inner surface of the drum and to the shaft, so that when the shaft rotates, the drum also rotates.

The pitch of the screw surface \((S)\) is equal to the diameter of the drum: \(S = D\).

The screw surface has 1.5 pitches along the length of the drum. The drum rotates in a trough, being immersed in liquid. Between the blades of the drum and the inner surface of the trough the clearance must be \(5\ \text{mm}\). The same clearance is between the rims of the drum and the end walls of the trough.

In the end wall of the trough on the discharge side (rear side) there is a pocket, from which the upper product is extracted by an elevator with perforated buckets (hole diameter 5 mm).

Through the same rear wall of the trough passes a grate inclined to the horizontal at 65°, the upper wide end of which is located directly under an annular launder with 12 compartments formed by radial perforated partitions (hole diameter—5 mm).

Adjoining the launder, with a clearance of 5 mm, is a baffle plate on the side of the elevator pocket.

In the end wall of the trough on the feed side (front side) there is a charging hopper and a pocket in which a float indicator of the liquid level is located.

The trough has two 3″ branch pipes, arranged as indicated in the sketch, and a 1″ cock for taking a liquid sample.

The direction of the helical surface must correspond to the feed of material from the front wall to the rear; the rotation of the drum, when viewed from the front, must be clockwise when the elevator pocket is located to the right and the grate for refuse to the left of the axis. The width of the blades for mixing the suspension is taken as \(=0.1D=300\) mm.

The capacity of the tubular transporter is determined by the formula [2].

\[ Q = 60 \frac{\pi \cdot D^2}{4} \, S \cdot n \cdot \Psi \cdot \gamma \ \text{t/hour}. \]

where \(D\)—diameter of the drum, m,
\(\quad S\)—pitch of the screw, m,
\(\quad n\)—number of revolutions per minute,
\(\quad \Psi\)—coefficient of filling,
\(\quad \gamma\)—bulk density of the material.

In the present case there will be movement of material upward at the liquid level (the floated part) and downward along the drum (the sunk part); depending on the specific gravity of the suspension and the beneficiation flowsheet, the upper product may be either concentrate or an intermediate fraction, while the lower part may be either an intermediate product or final refuse.

Taking the filling coefficient for the upper and lower layers at the lower limit, i.e., 0.2 [2], and assuming the bulk density of the concentrate equal to 0.75, and that of the middlings product—1.60, we determine the capacity of the machine according to the above equation.

For concentrate:

\[ Q_1 = 60 \frac{3.14 \cdot 9}{4} \cdot 3 \cdot 0.5 \cdot 0.2 \cdot 0.75 = 95 \ \text{t/hour}. \]

For middlings product:

\[ Q_2 = 60 \frac{3.14 \cdot 9}{4} \cdot 3 \cdot 0.5 \cdot 0.2 \cdot 1.6 = 203 \ \text{t/hour}. \]

If the amount of intermediate product fractions is, for example, 30% of the raw coal, then the total load of coal to be beneficiated on the machine will be:

\[ Q = \frac{95 \times 100}{70} = 135 \ \text{t/hour}. \]

The capacity for the sunk part (middlings product or refuse) along the helical surface must be coordinated with the capacity of the annular launder lifting the sunk part.

In the example considered, the amount of middlings is, in round numbers, 20 t/hour.

With the accepted dimensions of the separator, the area of the circular annulus with radii 1.5 m and \(1.5+0.3=1.8\) m will be:

\[ 3.14(1.8^{2}-1.5^{2})=3.1\ \text{m}^{2}. \]

The capacity of all 12 compartments of the trough, with a total area of 3.1 m², must be such as to discharge, in one hour at 0.5 revolutions per minute, 40 t of middlings with a bulk density of 1.6. Taking the filling coefficient of the trough compartments to be, for example, 0.5, we determine the necessary trough width \(x\):

\[ x=\frac{40}{3.1\cdot 0.5\cdot 1.6\cdot 0.5\cdot 60}=0.54\ \text{m}. \]

The concentrate is discharged from the separator by means of an elevator with perforated buckets, the lower part of which is mounted in a pocket on the rear wall of the machine. Experiments with the model have established that the suspension in the pocket remains stable during rotation of the drum, and the concentrate is kept here in a floating state.

In the example under consideration, the elevator must discharge 95 t of concentrate per hour.

If, on the basis of average standards for coal elevators, the bucket filling coefficient is taken as 0.5, the number of buckets per 1 running meter of belt as 4, and the speed of the latter as 0.4 m/sec, then the required bucket capacity \(W\) will be:

\[ W=\frac{95\cdot 1000}{0.75\cdot 0.5\cdot 0.4\cdot 4\cdot 3600}=44\ \text{l}. \]

An elevator of the corresponding dimensions can be fully accommodated within the possible dimensions of the pocket for the given dimensions of the machine.

The design of the separator may be carried out somewhat differently, namely: the rotating drum may be mounted not on a shaft, but on rollers, with a drive in the form of a gear rim and reduction gear, as is done in feeding pipes (tubular conveyors); however, in this case it would be necessary to reckon with heavy wear of the rollers rotating in the magnetite suspension.

The lower size limit of the coal beneficiated in a separator having a drum with 5 mm openings is assumed to be 10 mm, while the upper limit, generally speaking, is unrestricted; for very large sizes, only certain design changes will be required with respect to the depth of the annular trough, the size of the drum openings (to reduce its weight), the shape of the elevator buckets, etc.

The crushing of the lower product, which is possible to a certain extent in tubular conveyors, as well as in drum screens, is in the present case suppressed by the fact that the material moves in a heavy and viscous medium, as a result of which the force of gravity is reduced and impacts are softened; this is also aided by the very low angular speed of rotation of the drum. Damage to the upper floating product should obviously be practically absent.

Despite the simplicity of its design, the apparatus has a relatively high capacity, which does not decrease, with respect to concentrate, as the amount of intermediate fraction or rock (in general, the lower product) increases.

In contrast to this, in beneficiation in a machine with horizontal suspension flows, as established by M. V. Tsiperovich at Guba-

At the Zinn experimental installation, when the yield of middlings is more than 20%, the separator capacity drops sharply, since the velocity of the horizontal flows is insufficient for transporting the middlings [3].

An increase in the residence time of coal in the suspension of the tubular separator can be achieved in two ways—either by increasing the length of the drum without reducing capacity, or by reducing the pitch of the screw surface, in which case a reduction in capacity will occur, all other conditions being equal.

Testing of the separator in a semi-industrial installation is necessary in order to obtain production indicators.

References

1. Preobrazhenskii P. N. Beneficiation of coals for coking, p. 217, 1950.
2. Spivakovskii A. O. Conveyor installations. Part IV, p. 96, 1935.
3. Shprogge M. V. and Afonina K. B. Experience in the beneficiation of coals in a heavy suspension. Stal’, No. 1, pp. 18–28, 1951.

1. Presented at a session of the Leningrad Branch of the Academy of Sciences of the USSR (November 1951), at a conference at the Tomsk Polytechnic Institute named after S. M. Kirov (February 1952), and at the Technical Council of the “Kemerovugol” combine (April 1952). ↩

2. A drawing relating to the model. ↩