Process of separating intermixed divided materials
United States Patent 2348344

This invention relates to the separation of substantially dry, intermixed, divided materials whose constituent particles differ in density. In general, the invention will be described with reference to the separation of slate or other refuse from coal, but it will be understood that neither...

Higham, Holmes Colin William
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This invention relates to the separation of substantially dry, intermixed, divided materials whose constituent particles differ in density.

In general, the invention will be described with reference to the separation of slate or other refuse from coal, but it will be understood that neither the process nor apparatus is confined thereto.

Objects and advantages of the invention will be set forth in part hereinafter and in part will be obvious herefrom, or may be learned by practice with the invention, the same being realized and attained by means of the steps, instrumentalities and combinations pointed out in the appended claims.

The accompanying drawings, referred to herein and constituting a part hereof, illustrate one embodiment of apparatus for carrying out the invention, and together with the description, serve to explain the principles of the invention.

Of the drawings: Fig. 1 is a top plan view of a twin deck separator on which the improved process may be carried out; Fig. 2 is a vertical longitudinal section on line 2-2 of Fig. 1; Fig. 3 is an enlarged transverse vertical section on line 3-3 of Fig. 2; Fig. 4 is a transverse section on line 4-4 of Fig. 2; and Fig. 5 is a diagrammatic longitudinal section of a modified form of deck illustrating the operation of the invention on a bed of intermixed coal and refuse materials.

The invention is directed to providing an improved process for separation of such intermixed divided materials according to their differences in density. The process of the invention is applicable to either a sized or unsized feed of raw coal or like material to be separated. It has been found particularly effective in producing a clean, coal-free refuse product while at the same time providing a coal product free from refuse, without sacrificing the cleanliness of either end product and also without the necessity for removing a so-called middlings product which is commonly required in other types of separating processes without sacrificing the purity of either the coal or the refuse products.

One object of the invention lies in the provision of a combination of mechanical and pneumatic means, whereby both the efficient separation of the raw material and the discharge of the separated products is rendered substantially automatic and therefore independent of (1) fluctuations in the feed quantity of raw material from i55 zero up to a predetermined maximum, and (2) substantially independent of variations in the relative amounts of light and heavy particles, or, alternatively, of coarse and fine particles, or both, in the raw feed.

As to process improvements the invention primarily relates to that type of pneumatic separating operation wherein the raw materials are fed to the surface of an inclined, reciprocating, air-pervious deck where they undergo pneumatic stratification and the separating action and reciprocating movement are such that the upper strata of coal or lighter material travel downwardly to discharge at the lower end of the inclined deck, while the inferior strata heavy material travel upwardly in the opposite direction to discharge at the upper end of the inclined deck.

The improved process features primarily responsible for the improved separating action of the invention comprise generally the provision of a pervious deck or separating surface that is horizontal at right angles to the reciprocating or head motion but which is more steeply inclined to the horizontal towards the discharge of the heavy particles than it is towards the discharge of the light particles.

Cooperating with the aforesaid deck, the invention provides for the maintenance and control of a critical air pressure condition which is so related to the resistance of the air pervious deck that suitable and unique separating conditions are maintained at all times regardless of changes in the quantity and character of the raw feed.

For this purpose the invention provides a supply of air under pressure, control valves, an air chamber below the deck or separating surface, the said separating surface being relatively smooth on its upper surface and so designed that it diffuses and restricts the volume of air which escapes through its perforate surface by virtue of the pressure maintained within the air chest, the said restriction lying between two relatively close values.

For carrying out the aforesaid process improvements, the invention also provides certain improved mechanical means. One such improvement consists in the conversion of the smooth rotary motion of the driving head shaft into a reciprocating motion at the deck or separating surface, the said motion, being at a substantially greater angle to the horizontal than those between the separating surfaces and the horizontal, the motion being characterised by the fact that it is free from sudden drops or jolts.

Another object of the invention lies in the provision of means whereby the horizontal component of the reciprocating motion is substantially absorbed by a system of levers and torsion spring bars in the downward and backward stroke, and this energy is returned by the springs to the reciprocating mass during the forward and upward stroke, so that the relatively large and heavy masses involved in the separation of large quantities of raw material may be reciprocated, by way of example, through 1200 reversals per minute with an amplitude of %" without creating clearances in the driving head or link joints or other parts of the mechanism and without creating destructive secondary vibrations either at the separating surface or in the supporting structure, so that a continuously uniform mechanical agitation can be transmitted to the bed of particles as a whole and so that a continuously uniform traverse effect can be transmitted to the stratified heavier component of the mass. In this connection the present invention eliminates all auxiliary fittings such as deflecting riffles, overlays, and the like, to augment the traverse effect of the head motion whose influence on the separation varies as they become worn, and also eliminates.vanes or baffles placed above or below the pervious deck for the deflection of currents of air.

An important difference between the present invention and the prior art lies in the fact that prior machines acted much as jigging conveyors, operating continuously in removing the lower stratum of the bed irrespective of its composition, whereas in the case of the process described herein the constant traverse value is always present, but it only effects a removal of the lower stratum when said stratum is in a substantially pure condition and when its accumulation has created certain pneumatic and thrust conditions in the bed of particles. The removal of the lower stratum of heavy particles ceases automatically as soon as a limiting pneumatic condition is reached, the normal working bed of heavy particles remaining on the deck until more heavy particles are provided by the raw feed.

Prior machines have employed separating surfaces that were planes both in line with and at right angles to the head motion, but in the present invention the inclination of the deck is increased from a relatively small angle to the horizontal at its lower end to a steeper inclination at its upper end, because it has been found by experiment that substantial automatic operation cannot be obtained and continuously maintained on a plane surface.

It has also been found by experiment that said combination of deck angles is necessary-(1) to permit of the free flow of the stream of light particles off the lower end of the deck, and at the same time to permit of the retention of a bed of light particles of predetermined depth when the raw feed stops; (2) to permit of the formation of a relatively fluid reservoir of heavy particles having a head value that is to all intents and purposes hydraulic in its action, so that heavy particles are not discharged under no-load conditions but are (to use an analogy) pumped away by the head motion so soon as the accumulation of further quantities of stratified heavy particles create the necessary pneumatic and thrust conditions for the head motion to take effect.

It has also been found by experiment that both the value of the angle of the slope down which the light particles flow off the deck and the value of the difference between this angle and that up *which the heavy particles are traversed are critical, the differences of the order of plus or minus one degree being important in any given set of circumstances.

Further, it has been found by experiment that if the pervious deck is continuous inclined plane, the conditions under which separation can be obtained are too critical to give automatic operation, while on the other hand if the difference between the slopes of the two ends of the deck is too great the heavy particles will accumulate on its surface until they have damped out the desired pneumatic condition over a large area, after which the whole bed of particles will discharge rapidly at the upper end of the separator and the separation will then go permanently out of balance.

Machines constructed according to the prior art were generally provided with decks of high air permeability; for example being decked with woven wire cloth, that is to say, with a cover having not less than 25% of its area available for the escapement of air from the underside. Alternatively, the air-pervious deck cover consisted of a perforated metal plate having a relatively high percentage opening, for example, between 25% and 40% of the plate area. Because of this fact such prior machines usually employed baffles or deflecting vanes beneath the deck surface so as to direct and control the distribution of the air blast passing upwardly through the deck.

In the process as carried out in accordance with the present invention, the air-pervious deck cover is so constructed as to offer a greater resistance to the passage of air than was the case in the prior art, for example, deck covers with an effective opening of less than 2% of the plate area are usually necessary in the treatment of the finest sizes of material, and it has been found by experiment that the automatic separation as disclosed herein can only be obtained on decks offering considerable resistance to the passage of air from the pressure condition obtaining in the air chest at the underside of the deck. The minimum degree of restriction must be such that the velocity of escapement of the air is rendered uniform over the whole deck area when empty without the use of deflecting vanes, or the like, although a greater degree of restriction than this limiting value is usually employed.

Apart from acting as a diffuser, the restriction imposed by the deck surface upon the escapement of air through it must be sufficient to create a pressure condition in the air chest greater than that required to break down the maximum thickness of coal bed to a fluid condition and to maintain the bed in this state while permitting only a relatively small variation in the volume of air passing through the bed as its resistance varies from full load to no-load conditions, or, alternatively as its resistance increases or decreases due to size or density segregation in the raw feed. 6 In carrying out the separation of refuse from coal, by way of example, on pneumatic separators having a deck shaped and mounted as shown in the accompanying drawings, three different conditions have been observed as a result of experiment with changes in the air-pressure conditions, as follows: (1) With machines decked according to the prior art, that is to say, with a ratio of opening to deck area of approximately 1:4, it is found that slight variations in the rate and/or grading and/or refuse content of the raw coal feed will, by way of example, cause the resistance of the bed to fall at the lower end of the separator where it is covered by a bed of clean coal particles.

When this happens, the deck--offering relatively little resistance to the passage of the air--permits of a prompt and greatly increased escapement of air, with the result that the pressure condition in the air chest falls and (with a given air-fan output) is no longer sufficient to maintain the bed of refuse (which occupies 30-50% of the deck area) in a reasonably fluid condition, so that partly by virtue of the increased proportion of its mass that registers on the deck surface, and partly due to the fact that friction between the deck and the lowermost layer of refuse particles as well as that between the individual particles themselves is increased, the whole bed, including the overlying coal, takes the traverse of the head motion and is rapidly discharged at the upper end of the deck surface.

It will be understood that so long as the lower end of the deck is free to vent air against a low resistance, the separation will not recover and the separator will continue to act as a jigging conveyor, discharging the whole of the feed at the upper or refuse discharge end.

(2) With similar machines and raw coal feed, but with a deck cover, for example, with a ratio of opening to deck area of 1:10, similar conditions obtain though not to the same degree. The discharge of refuse will be intermittent, occurring at an abnormal rate for a few seconds, after which the fringe of the refuse bed will recede from the refuse spillage edge of the separator and the whole refuse bed will then slip down the deck towards, and sometimes over, the clean coal spillage edge, leaving the upper end of the deck surface exposed. This gives a relatively free vent to the air at the upper end of the deck, which, in turn, causes a slightly reduced pressure beneath the bed of refuse; in consequence it again takes the traverse of the head motion and gives a brief and abnormally heavy discharge before slipping down the deck once more.

It will be understood that such conditions are undesirable, because the optimum conditions for prompt and efficient stratification cannot be maintained continuously under such fluctuating bed conditions, and, in-consequence, the purity of the separated products will be variable and generally unsatisfactory. In addition, the tendency for the refuse to slip down the bed and pass over with the clean coal, as well as the tendency for clean coal to be dragged over with the intermittent refuse discharge, is considerable.

(3) With a similar machine and raw coal feed, but with a deck cover, for example, with a ratioof opening to deck area of 1:20, that is to say, in accordance with the present invention, it has been found by experiment that the separation will remain continuously in balance despite any fluctuations in the rate of feed between zero and maximum and irrespective of any variations that have so far been observed as regards the grading of refuse content of the raw coal feed.

It is found that under these conditions any reduction in the rate of feed causes, of necessity, a reduction in the depth of the stream of clean coal flowing downwardly over the inclined lower end of the separator deck, with the result that the resistance of this bed to the passage of air is decreased.

But the resistance of the bed of coal is no longer the main controlling factor. It is now the deck, and the restriction that it offers is so great that although it will transmit sufficient pneumatic energy to maintain the bed in a fluid and free stratifying condition it will not pass a sufficient excess of air with a bed of lower resistance either to blow the bed off the deck altogether or to permit of a damaging reduction in pressure beneath that portion of the deck which supports the bed of refuse.

In consequence, the refuse bed on the more steeply inclined part of the deck is maintained in a sufficiently fluid condition for it to maintain its hydraulic head effect on the refuse lying on the flatter portion of the deck and also in such a fluid state that it is not affected by the traversing influence of the head motion. Thus under no-load conditions the bed remains stationary, that is to say, that it does not travel up or down the inclined surface and neither clean coal nor refuse is discharged, although the head motion is being imparted to the deck.

If pre-cleaned coal is now substituted for the raw feed, it is all discharged at the lower or clean coal discharge end of the deck, and if refuse is fed to the machine it stratifies below the clean coal on the deck surface slightly increasing the length and depth of the lower end of the refuse bed on the separator.

As the refuse bed under no-load conditions is in balance with the combined mechanical and pneumatic forces, tending respectively to cause it to discharge at the upper end of the deck or to slip back down the inclined supporting surface, the addition of this further refuse and the consequent extension of the refuse bed on to the flatter portion of the deck upsets this balance because: (a) A portion of the refuse bed now rests on an area of the deck which is flatter than the angle of slip under the uniform pneumatic conditions that have been constructed, the head motion therefore traverses it towards the refuse dis,0 charge end of the separator, with the result that it displaces an equivalent amount of the balanced refuse bed over the discharge edge of the separator.

(b) In effecting this, the previously balanced refuse bed must become slightly deeper before 45 it can discharge over the shale spillage edge, and this again has a slight pneumatic effect in increasing its traverse action.

It will be understood that the bed of refuse is in equilibrium until an additional thrust value 60 is imposed upon it by freshly stratified refuse, and, further, that such thrust must be transmitted throughout the length of the refuse bed so as to permit of the discharge of refuse from the separator. It will also be understood that 55 such transmission of energy can only be effected over a relatively smooth deck surface, and that any teeth, steps, overlays or other obstructions on the deck surface such as were employed in the prior art in order to traverse out the refuse, co would, with the present invention, interfere with the smooth and practically hydraulic transference that is obtained by the process as described herein.

It will also be understood that an essential 65 discovery of this invention lies in the maintenance of a reserve of pneumatic energy in the form of a pressure condition beneath the pervious deck in combination with means for limiting the rate at which this reserve of energy in 70 the form of pressure can be dissipated in velocity form, thereby ensuring that the pneumatic conditions under which the stratification of the raw product and the removal of the separated products shall not vary except between such nar75 row limits as to permit of prompt and continuous stratification and separation irrespective of such fluctuations in the quantity and quality of the feed as are commonly met with in the art.

It will also be understood that the invention is not confined to the use of a deck cover having a permeability ratio of 1:20. It has been found by experiment, for example in the separation of coal from refuse that: (1) The maximum percentage opening in the deck cover necessary to stabilise the separation is a function of the square root of the diameter of the largest particle.

(2) When round openings are employed their diameters should preferably lie between 10 mm. and 1 mm., depending upon the size of material to be separated.

(3) The distributions of the perforation should be reasonably uniform.

It will be understood that the foregoing general description and the following detailed description as well are exemplary and illustrative of the invention but are not descriptive thereof.

Referring now in detail to the present preferred embodiment of the invention illustrated by way of example in the accompanying drawings, the invention provides an air pervious deck I as hereinabove described, which deck together with an air chest or air chamber 2 mounted beneath it and fixed to it, is adapted to be reciprocated longitudinally upon a stationary supporting frame 3. The air chest is flexibly connected to a fan or other source of pressure air by means of one or more evas6s 6. The reciprocatory motion of the deck and air chest with respect to the frame is made possible by links 7 pivotally mounted at their lower ends in rubber bushings carried by the frame 3 and connected at their upper ends through similar bushings to the under surface of the air chest 2.

Mechanical energy from any suitable source is transmitted to the mechanism through a suitable pulley 10, mounted upon a shaft II, supported in ball bearing blocks 12, and upon which are mounted ball bearing eccentrics 13, the whole being mounted in a casing 14.

The eccentric motion is transmitted to the separator by means of eccentric rods 15, terminating in rubber bush fittings 16, the line of said motion being such that it passes through, or as nearly as possible through, the center of gravity of the reciprocating mass.

Torsion spring bars 17 are rigidly connected to the air chest structure 2 at their inner ends.

They are fixed in pairs coaxially with the centers of oscillation at the upper ends of the supporting links, and their outer ends are rigidly connected to the upper ends of levers 9, which terminate at their ends in slots running radially with the clamping bolts 18 in the lower ends of the supporting links.

In order to set up this torsion spring balancing arrangement, bolts 18 are slackened and the eccentric shaft is rotated, until the air chest is nearly at the end of the forward and upward stroke. Bolts 18 are then tightened up so that the levers 9 are fixed rigidly to the inner links 7, the contacting surfaces being roughened so as to eliminate the possibility of slipping. The mechanism will then settle down so that it is almost on the bottom and rearward center, thus placing a torsional stress on the spring bars 17.

In actual operation these bars are stressed during each cycle and the energy stored in them is released during the forward and upward stroke, thus eliminating reversal shocks in the mechanism generally, and more especially reversals of stress in the eccentric rods 15. Means are provided for maintaining a suitable and preferably substantial depth of bed of materials on the deck under no-load conditions.

For this purpose the raw material to be treated is fed uniformly across the width of the separator along a line at right angles to the head motion, and approximately one-third of the length of the deck away from the refuse discharge. The source of feed may be a surge bunker, although, provided that the rate of feed does not exceed a predetermined maximum, the raw material may be handled in stream and the surge bunker eliminated.

Adjustable spillage edges 4 and 5 are provided at the upper and lower ends respectively of the deck so that the depth of the bed under no-load conditions can be set to any desired value.

Referring now to Fig. 5, the distribution of the refuse and coal particles is diagrammatically shown upon the pervious deck under noload conditions, and at the instant of recommencing the feed. It will be understood that this drawing is purely diagrammatic and that it is not to scale. It will be seen that the bed of refuse particles on the more steeply inclined portion of the deck terminates in a shorter length of bed resting on the flatter portion of the deck and whose length is shown by the dimension X. It will be understood that on feeding further raw material to the separator, this is promptly stratified and the heavier particles sinking to the deck surface give a length of bed greater than X, and, in consequence, have a greater thrust value towards the spillage edge 4 than previously. In consequence, refuse is discharged continuously until such times as the length of refuse bed on the flatter portion of the desk returns to the length X, when conditions are again balanced.

The lighter coal particles stratifying above the refuse are maintained in fluid bed form by the air pressure in combination with the reciprocating motion, and flow in stream form continuously down the angle a over the spillage edge 5. 0 Means are provided for altering both angles a and b simultaneously by adjusting the slotted fixings 19 and 20 in Fig. 2, and either angle can be altered independently by loosening, adjusting and reclamping the two or more sections of the deck between the upstanding walls of the air chest.

As shown by way of example in Fig. 2, the air pervious deck I is formed as three panels wherein the intermediate panel is fixed, while the lower or forward panel la is pivoted at 21 to vary the slope of said panel with respect to the intermediate section I (which conforms generally to the area marked "X" in Fig. 5), while the upper or rearmost panel lb is pivotally connected to panel 1 at 22 thereby to be positioned to form the steeper angle b as diagrammatically indicated in Fig.

5. The mounting of the panel lb with respect to the deck frame 25 and the intermediate dividing wall 26 will be clear from Fig. 3. The invention is exemplarily shown as applied to a double or twin deck construction wherein one-half of the deck may be operated independently of the other half and upon materials having different characteristics, if desired, and the construction thereby permits of setting the various deck panels at different angles as regards the operations of the two side-by-side decks.

It will be understood that the deck may be formed as shown in Fig. 5, that is, with two fixed slopes corresponding to the angles a and b or that a number of variably positionable panels similar to those in Fig. 2 may be used in greater number so as to vary the angularity of different parts of the deck at different ratios, without departing from the spirit of the invention.

It will also be understood that presized or substantially unsized coal may be treated, that either one or both of the products from the preliminary separation may be subject to screening and subsequent retreatment, or to retreatment without such screening, and that the process and apparatus may be employed in the cleaning of coal or in the separation of other substantially dry and divided materials in conjunction with other processes and apparatus known in the art.

As has been previously stated, the permeability of the deck I will in all cases be so restricted as to set up the air-pressure condition hereinabove described. It will be understood that the showing of an air pervious deck in the drawings hereof is entirely diagrammatic and is not intended to be representative of the actual ratio of air permeability.

It will be understood also that the decks may be constructed of cup-punched plate instead of plain punched plate, in order to obtain a more efficient transformation of the pressure condition beneath the deck into the velocity condition above it; the ratio of the height of the cupping to its diameter being of the order of 1:4, the deck surface is still maintaining in a smooth condition, and it has been found by experiment that roughening to this extent does not interfere with the separation according to the present invention.

Also it will be understood that when the separating surface consists of two or more parallel channels divided by upstanding walls, as shown, that each of said channels may be equipped with decks having different percentage openings and/or different inclinations and/or different depths of bed, and that they may operate with different air pressures so that two or more different qualities or sizes of coal may be separated simultaneously on a single machine.

The invention in its broader aspects is not limited to the specific mechanisms shown and described but departures may be made therefrom within the scope of the accompanying claim without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is: The process of separating intermixed divided materials which comprises feeding said materials to an inclined air-pervious surface between the ends thereof, imparting a reciprocating head motion to said surface to impel materials toward the 2upper end thereof, passing air under predetermined pressure upwardly through said surface and the bed of materials formed thereon to stratify and fluidize said materials, causing lighter materials to travel downwardly longitudinally to the lower end of the surface to discharge by in25 dining the surface at a predetermined angle between the feed-on point and the lower end of said surface, opposing the opposite upward travel of heavier materials by inclining the surface at a greater angle between the feed-on point and the 30 upper end of said surface causing stratified heavier materials to discharge at the upper end of said surface and maintaining the air escapement through the bed substantially constant under changes in the air resistance of the bed of mate35 rials by providing throughout the surface an airpermeability which is relatively low compared to that of the bed under maximum load conditions, and by maintaining a substantial air pressure beneath said surface, and constraining all mate40 rials to discharge at the ends of the surface by preventing lateral movement and escape thereof.