WO2000031309A1

WO2000031309A1 - Method of recycling swarf into shot blasting material

Info

Publication number: WO2000031309A1
Application number: PCT/GB1999/003841
Authority: WO
Inventors: Antonino Giorgio Cacace
Original assignee: STELAX INDUSTRIES Ltd
Current assignee: STELAX INDUSTRIES Ltd
Priority date: 1998-11-19
Filing date: 1999-11-17
Publication date: 2000-06-02
Anticipated expiration: 2001-05-19
Also published as: AU1394100A

Abstract

A method of recycling swarf into shot for use in shot blasting machines is disclosed. Swarf, especially swarf composed of engineering steel, is separated from large objects in a trommel [10] and is cleaned of contaminants. Oil and water is removed by passing the swarf through a rotary kiln [28]. The swarf initially comprises pieces of random varying size which are passed through at least one crusher [18, 20, 34, 80] which progressively reduce the size of the pieces and a grading apparatus [38] which separates the pieces of different size into groups at least one of which comprises pieces which can used as shot. In at least one of the crushers [80] the swarf pieces may be subjected to a shaping operation in which the swarf pieces are rounded.

Description

METHOD OF RECYCLING SWARF INTO SHOT BLASTING MATERIAL

FIELD OF THE INVEN ION

This invention relates to shot blasting material It is an object of the invention to provide a use for scrap steel swarf BACKGROUND OF THE INVENTION

The applicant believes that the terms swarf and 'engineering steel' will be understood by persons of experience in the art. However, it is convenient to clarify the meaning intended to be ascribed to these and certain other terms which are used in this specification in case any of the terms is found not have a known clear meaning

Thus 'swarf is intended to include at least the off cuts produced by turning, boπng, shaping, drilling and milling operations on steel or other metals where such off cuts can be processed by the methods outlined herein into particles which can be used as shot in at least some conventional shot blasting machinery However, swarf comprising the off cuts from a variety of other metal working operations including some stamping and punching operations may also be suitable.

The term "engineering steel" is intended to include at least those low alloy steels which are commonly subjected to machining operations, including mild steel (a term which itself includes carbon steel), forging steel and axle or shaft steel It is the not the intention of the applicant that the scope of the present invention is necessarily restricted to the use of swarf composed of engineering steel For example, swarf produced by machining operations on a suitable grade of stainless steel or other metal including steel castings and free cutting steel may be suitable to be processed into shot by the techniques described herein However, because of its low cost and wide availability- the invention is likelv to have its most common application to swarf composed of "engineering stee!^" as discussed above

In its raw state, scrap steel swarf produced in the course of machining operations as discussed above is in the form of elongate- more or less springy pieces Typically, in their original state, these pieces vary randomlv between a few millimetres and many metres in length When used in relation to swarf in this specification- the term ^"chips is intended to refer to the original pieces which make up a mass of swarf. As distinct from this, the term "particles^" is intended to refer to pieces which make up a mass of swarf which has been processed in one or other of the ways described herein so that the average size of the pieces is generally smaller and more uniform as compared to the size of the chips. Furthermore, there are present in raw swarf significant quantities of impurities such as the oils which are used in the machining operations, water, oxides, mill scale and dirt. It is well known that raw swarf is not in optimum condition either to be transported or to be fed to a furnace for remelting which is the usual method of recycling swarf. Consequently raw swarf is commonly subjected to a centrifuging operation to remove some of the oil and water and is also compacted into bundles before it is delivered to steel mills for remelting. SUMMARY OF THE PRESENT INVENTION

According to the present invention, there is provided a method of recycling swarf in which raw swarf is treated to produce a mass of swarf comprising swarf particles which are of selected size and which are suitable for use as shot in a shot blasting operation. According to one aspect of the invention the raw swarf comprises swarf chips at least some of which are reduced in size to produce the mass of swarf particles. The treatment of the swarf may thus include pieces of swarf to one or more size reduction operations, one or more grading or sorting procedures, or a combination of both. In one form of the invention at least some of the swarf chips are reduced in size to produce a mass of swarf pieces and at least some of the swarf pieces are reduced in size to produce the mass of swarf particles.

In one form of the invention, the swarf chips and/or the swarf pieces are reduced in size by passing the swarf through at least one hammer mill. According to one aspect of the invention the swarf pieces or the swarf particles are subjected to a shaping operation in which they are rounded.

The swarf is advantageously but not necessarily essentially comprised substantially of engineering steel.

Further according to the invention the method includes the step of using the swarf particles as shot in a shot blasting operation. The scope of the invention includes shot comprising swarf particles produced by any of the inventive methods set out herein

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the invention are discussed in the following description of examples of the invention and with reference to the accompanying drawings in which Figure 1 is a block diagram showing in schematic form successive stages in a process for producing a product composing swarf particles which can be used as shot in a shot blasting operation, Figure 2 is a tracing of a photograph of an actual sample of typical swarf particles which are suitable for use as what is referred to herein as coarse grade shot, Figure 3 is a similar tracing of larger swarf particles produced in the process,

Figure 4 is a tracing of swarf particles from the same batch as those shown in Figure 3 but after being conditioned as described herein, Figure 5 is a somewhat schematic sectional view of a crusher,

Figure 6 is a plan view of a small part of the inner face of a grid of the crusher shown in Figure 5, developed so as to be viewed in the flat,

Figure 7 is an enlarged cross-sectional view on arrows A-A of a single perforation in the grid; Figure 8 is a view, similar to Figure 6, of a grid with perforations in the form of tapered slots. Figure 9 is a somewhat schematic side view of a grading apparatus,

Figure 10 is a view, similar to Figure 5. of a conditioning crusher used in a modification of the process shown in Figure 1 , and Figure 11 is a schematic view of the profiles of particles of conventional cast shot and of shot produced by the methods of the present invention DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

It should be understood that it is not intended that the scope of the invention be limited by matter which appears in the discussion which follows unless a contrary intention appears from the context

To prepare typical shot blasting mateπal of the invention, swarf in the form of scrap steel chips is used In the present example, the swarf comprises off cuts of EN8 steel produced in the course of turning or other machining operations in a large scale machine shop The selection of scrap steel swarf of this grade is made, in the present instance, principally on account of the reliability and convenience of supply and uniformity of quality It is advantageous but not essential if chip breaking tools are used in the machining operations in which the raw swarf is generated Use of such tools has the result that the chips are of substantially shorter average length than would otherwise be the case This facilitates handling and transportation of the raw swarf Such swarf is optimally in a condition known as 'shovelling' grade However, even shovelling grade swarf composes many chips of substantial length which m practice become entangled with each other and form large bundles Swarf which is composed of many such bundles is known as 'bushy^" grade swarf

The raw swa^r. is usefully subjected to a centnfuging operation to remove some of the entrained o- and v. -αer This may be earned out at the locality where the raw swarf is generated and is an advantage in carrying out the present invention but is not essential As received-, the swarf in the present example is first passed through a trommel 10, a known device composing a cyhndocal god which rotates about its longitudinal axis This axis is inclined to the hoozontal so that swarf which is fed into one end of the trommel (typically by a front end loader) passes through the trommel under gravity as the trommel rotates Round holes are formed in the god The trammelling of the swarf loosens any bundles of bushy swarf and frees unwanted large objects such as bars and bolts which may damage the crushers or other equipment which is used to treat the swarf after it has passed through the trommel These objects are removed by hand The bulk of the swarf is discharged from the lower end of the trommel but the smaller chips in the swarf drop through the holes in the god onto a belt conveyor for subsequent treatment as will be described This and the other belt conveyors described below are conventional and need not be described in detail In the present example the holes in the trommel god are typically 30-40 mm in diameter The swarf which emerges from the discharge (I e the lower) end of the trommel 10 is fed by a belt conveyor to a pomary crusher 18 The construction of a typical crusher is described below In the pomary crusher, the chips are broken down into pieces the great majooty of which in the present example are not more than 100 mm in length In the present example, the crushed swarf which emerges from the primary crusher 18 is fed by a belt conveyor to an intermediate crusher 20 The swarf may sometimes be fed instead to a storage bin 22 before being fed to the intermediate crusher 20. Furthermore, the smaller chips which were sorted from the raw swarf in the trommel 10 are also fed to the intermediate crusher 20. In the crusher 20, the swarf is further broken down into pieces the great majority of which typically do not exceed 20 mm in length. In the present example, these pieces are fed by a belt conveyor to a storage bin 26 and thence by another belt conveyor to a rotary kiln 28. In the kiln 28 the oil and water in the swarf are removed. In the present example, the kiln is of the forced draught, gas fired type. The kiln desirably operates under a mildly reducing atmosphere at a temperature of 500-800° C since it is considered preferable to vapoose rather than burn the oil off. Because of the amount of oil and water which occurs in the swarf is variable, the kiln temperature is preferably automatically controlled. The gases from the kiln are fed to an afterburner where combustibles in the gases are burned off. The heat from the gases may be recovered and returned to the kiln so that very little additional fuel is required for the overall process of removing the moisture and oil from the swarf. The waste gases from the afterburner are scrubbed and cleaned before being discharged to atmosphere. The kiln, afterburner and scrubber are of conventional construction and need not be described in detail here. From the kiln, the swart^" is fed in the present example to a vibrating table 30 where it is spread out to facilitate cooling to about 100° C At this stage the swarf still contains a lot of dust made up principally of iron oxides and the residue (soot) of the vaporised oil. From the vibrating table the swarf is fed by means of a conventional bucket elevator to a final stage crusher 34 In the crusher 34, the pieces of swarf are further reduced in size to produce a product comprising steel particles the greatest dimension of the majooty of which does not exceed 8 mm. Most dust still adherent to the pieces of swarf is knocked off in the crusher 34. This dust is extracted by an extractor fan. This fan is also able to remove very fine particles of swarf which would not be useful when the product is used as shot in a shot blasting operation. From the final stage crusher 34 the swarf is fed by a belt conveyor to a grading apparatus 38 shown in Figure 9. In the present case this is a three stage trommel, comprising four cylindrical grids 39, 40, 42, 44, each of 1.25 metres diameter, 1.5 metres axial length and 3 mm thickness. The grids are disposed end to end along their common longitudinal axis which, like the axis of the trommel 12, is inclined to the horizontal at an angle equal in this case to 3.6°. Holes, which in this case are round, are formed in each grid. In the present example, the trommel rotates at a speed of 18 rpm. The swarf discharged from the final stage crusher 34 is fed into the upper end 35 of the grading trommel and, as the trommel rotates, gravitates first through the initial grid 39 the holes in which are of 1mm diameter. The purpose of this grid is to remove any dust, soot and any other very fine impurities remaining in the swarf as well as swarf particles of less than 1 mm nominal size. These are considered to be too small to be of use as shot. The matter removed by the grid 39 is collected and dumped. After passing through the grid 39, the swarf particles remaining in the grading trommel pass to the first stage grid 40 the holes in which, in the present case, are of 2 mm diameter. They then pass into the second stage grid 42 which has holes which are of 3 mm diameter, and finally through the third stage grid 44 which has holes of 4 mm diameter. Fine particles of swarf (i.e. particles of 1-2 mm nominal size) which can be used as shot are first separated by the first stage grid from the larger particles. Similarly, particles of 2-3 mm nominal size are separated from the larger particles by the second stage grid 42 and particles of 3-4 mm nominal size are separated from any larger particles by the third stage grid 44. In each case the graded swarf particles drop through the holes in the respective grid as the swarf passes therethrough. The particles which are too large to pass through the holes in the third stage grid are discharged from the lower end 45 of the grading trommel. The respective graded particles drop onto belt conveyors which take the particles to magnetic separators 46, 46', 46" of known kind where, in each case, any non magnetic material is removed from the swarf. From the magnetic separation stations the swarf particles are taken by belt conveyor to bagging stations 48, 48', 48" for bagging and ultimate use as fine, medium and coarse size shot blasting material.

Figures 2 illustrates the typical appearance of swarf particles which are intended for use as coarse size shot 56, as such particles emerge from the magnetic separator 46". The size of these particles may be judged from the scale, which in Figure 2 (as in Figure 3 and 4), is graduated in millimetres They have been found to function well as shot despite they fact that they vary somewhat in shape and mass The appearance of the particles emerging from the magnetic separators 46, 46' is similar but their size is correspondingly smaller It has been found by testing that the particles which emerge from all of the magnetic separators 46, 46', 46" function satisfactooly as shot when used in conventional shot blasting machinery of the wheelabrator type shown schematically at 49

When these tests were first conducted, because of concerns about excessive wear or damage to the machinery by the material, a shot mixture was prepared by mixing a quantity of the fine grade particles with a quantity of commercially available spheocal cast steel shot which was known to work satisfactorily in the machinery being used The initial mixing ratio was 1 part by weight of fine grade particles to 1 part bv weight of cast steel shot It was found that no damage was caused to the machinery when the initial shot mixture descobed above was used Shot is of course recycled through conventional shot blasting machinery and, as use progressed, it was also found that the fine grade particles in the shot mixture became progressively more rounded and resembled more closely the pellets which made up the cast shot Thereafter, further quantities of the fine grade particles were added to the mixture as it was reused m the conventional way Ultimately, so much of the fine grade particles had been added to the mixture that very little remained of the cast shot originally present Further sets of tests which were substantially identical to the tests in the first set were earned out using the medium grade and coarse grade particles as shot, with similar results

These tests have been repeated often All three grades of swarf particles have been found to function satisfactorily as shot in shot blasting operations

The above descobed methods of progressively adding the three grades of swarf particles to conventional shot were adopted out of caution In further tests, shot blasting machines have been charged with shot comprised purely of fresh swarf particles of each grade The qual of the shot blasting has been satisfactory No damage has been caused to the machinery in these tests and no evidence of excessive wear has appeared in the machinery to date The swarf which is discharged from the end of the grading trommel 38 comprises oversize particles, I e particles of greater than 4 mm nominal size Figure 3 illustrates the appearance and size of typical such oversize particles 58 It should be understood that the particles, in the dimension perpendicular to the drawing, are generally flatter than may appear from the drawing In the condition illustrated the particles 58 are too large to be used as shot in the machinery which has been used to date However, it may well be that the particles 58 may be suitable for use as shot in other shot blasting machinery If this is found to be the case, the particles can be passed through a magnetic separator 46"' and bagged at a bagging station 48"' for subsequent sale and use as shot m a shotblaster 49

It has however been found that, when treated in a conditioning crusher as descobe below, the particles 58 can be rendered suitable for use as shot Referong now to Figures 5 to 8, there is shown a crusher 1 10 which could serve as any of the crushers 18, 20, 34 discussed above These crushers are of conventional design and are described here merely to facilitate understanding of the invention The crusher 1 10 comprises a hammer assembly including a rotatable shaft 1 12 driven by a motor (not shown) A seoes of radially disposed arms 1 14 are mounted on the shaft Each arm is provided with a forked outer end carrying a hammer 116 The hammer composes a cyhndocal body 1 18 having teeth 120 on its outer face and a central passage 122 A pin 124 is mounted between the outer ends of the arm and passes with substantial clearance through the passage 122 By this means the hammer is free to rotate on the arm and moreover is capable of a substantial degree of movement in the radial direction on the arm The crusher further comprises an upper casing 136 which overlies the hammer assembly and is provided with a first aperture 138 through which swarf is fed into the crusher The casing 136 may also be provided with a second aperture 140 opening to a duct 142 in which a fan, not shown, is mounted Before being fed to the crusher 1 10 the swarf may have been cleaned and or crushed, as descobed above A certain amount of ash, oxide and soot will have been formed in this process Furthermore oxides and other contaminants are released from the swarf in the crushing operation A lot of dust may thus be evolved in the crusher ! 10 and this is removed by the fan through the duct 142 Baffles may be mounted in the duct to prevent unwanted entrainment of smaller pieces of swarf with the dust which is extracted by the fan A separator such as a cyclone may also be mounted in the duct 142 to separate the solids from the air A lιndπcal g d 126 is located below the hammer assembly The god partiallv surrounds the hammer assembly and is positioned so that when the hammers are in their extreme outer positions on the am s 1 14 there is clearance between the hammers and the inner face 128. This clearance is typically of about 30 mm in the first stage crusher but reduces to about 15 mm in the intermediate crusher and 4 mm in the final stage crusher. The grid is provided with many perforations 130. The perforations are in the form of cylinders or slots which in the present case are advantageously tapered as described below although the invention may also be performed with perforations of parallel sided form. In the present case the ends of the perforations at the inner face 128 of the grid are of smaller cross sectional size than the ends at the outer face 132. When in tapered cylindrical form, the perforations are in fact frusto-conical, having longitudinal axes of symmetry 134. Figure 8 illustrates a grid with perforations in the form of elongate slots 150 comprising walls 152 which are tapered. A sectional view on aoows B-B in Figure 8 would depict the same as what is shown in Figure 7. The slots 150 are disposed in parallel columns in the grid, the slots in one column being offset or staggered from those in an adjacent column. Swarf which enters the crusher is broken down into smaller pieces by the hammers as they rotate. The pieces are driven through the grid when they are reduced to a size which enables them to pass through the perforations.

The degree of initial size reduction in the primary crusher is limited in that, if the perforations in the grids are too small, oil and dirt in the swarf tend to clog the perforations and block them up completely. This ultimately causes the crusher to stall as too much resistance is offered by the uncrushed swarf which builds up in the crusher. This can also happen if, again due to size and number of perforations, the rate of passage of the crushed swarf through the perforations in any of the crushers is too slow. The size and shape of the perforations 130 is dependent on what is fed to the crusher in question. If the crusher serves as the primary crusher 18 as discussed above which receives raw swarf, the perforations 130 typically are elongate, in the present case being of size 100 mm by 25 mm. The applicant has found that the smallest practical size for the perforations in the primary crusher using swarf from turning operations is 50 x 25 mm. Perforations of smaller size lead to blockage and stalling of the crusher for the reasons given above. The primary crusher is the most vulnerable to this problem because of the large bundles of bushy swarf which require a considerable amount of hammeong before they are further broken down If the crusher is the intermediate crusher 20 as discussed above, the perforations will typically be round and have a diameter (indicated at Dl ) of 20 mm at the inner face 128 and a diameter (indicated at D2) of 24 mm at the outer face 132 of the god The intermediate crusher is not prone to stalling on account of the presence of bundles of bushy swarf as these have been broken down in the pomary crusher However, because the swarf in the intermediate crusher is still wet and dirty, there remains a tendency for the perforations to become blocked The applicant has found that, for the intermediate crusher, the perforations are advantageously cylindocal and the smallest practical size thereof is about 20 mm diameter

On the other hand if the crusher is the final stage crusher 34 as discussed above which receives cleaned swarf composing relatively small pieces, the perforations will again typically be round and in the present example diameter Dl is 8 mm and diameter D2 is 12 mm The perforations in the final stage crusher can be smaller than m the upstream crushers because there is no danger that they will become blocked from wet dirt as the swarf has passed through the drying kiln and most of the oil, water and other dirt has been removed. What governs the size of the perforations in the final stage crusher is the rate of throughput of swarf If this rate is too low (due to the size or number of the perforations being too small), a build up of swarf in the crusher may occur, leading it to stall If the rate is too high (due to the size or number of perforations being to large), the average number of times a piece of swarf is impacted by the hammers is reduced Impact action on the pieces of swarf is desirable as the pieces thereby tend to be cold forged to a rounded shape This is advantageous for reasons discussed below In the example shown in the drawings the gods of all of the crushers are of abrasion resistant steel and are 60 mm thick although thinner have also been used It is considered in general that the angle of taper of the perforations should be about 3° from the axis 134 The tapered form substantially reduces the tendency of the perforations to become blocked by the swarf pieces The gods are typically 1 5 metres in diameter and 1 5 metres long The hammer assemblies typically rotate at 500-800 rpm driven by 350 kW electric motors The actual shape of typical swarf^" particles produced in the process discussed with reference to Figure 1 is shown in Figures 2 and 3. Before the particles have been used as shot, their cross sectional shape is generally more rectilinear as compared to conventional cast spherical pellets of shot. These differences in shape are illustrated in Figure 1 1. The profile of a typical particle of shot produced by the methods described herein is shown entirely schematically in solid outline at 160 while the profile of a pellet of typical conventional cast shot is shown in dotted outline at 164. It can be seen that the particle 160 has relatively sharp edges 168 or corners due to its more rectilinear shape.

Certain disadvantages have arisen in using the swarf particles as shot. In the first place, the shape of the pellets of the conventional cast shot has remained unaltered for many years and some operators of shot blasting equipment are reluctant to use shot of any different shape. Second, the process by which the particles 160 are produced has the result that they vary in size and mass about an average, some being smaller and some being larger than the average size. Furthermore, because of its rectilinear shape, the length of a typical particle 160 will be greater than the diameter of a pellet 164 of equal mass. Shot blasting machines are equipped with sieves for sieving the shot with which the machines are loaded. This can have the result that a significant number of the particles 160 of a given nominal size may not pass through the sieves of certain types of shot blasting machines designed to use pellets 164 of the same nominal size. Furthermore, some users may perceive shot of rounded shape and smaller size to be less likely to cause damage or excessive wear to shot blasting machinery, although to date no such damage or wear has shown itself.

Shot particles having a rounded shape are likely to produce better results or be more efficient to use in some circumstances. The kinetic energy of a piece of shot in a shot blasting operation is "Λmv² where m is the mass and v is the velocity of the piece of shot. A spherical piece of shot is likely to have a greater velocity than a piece which is less round. In particular, a relatively flat or long piece of shot may be subject to an aerodynamic effect which reduces its kinetic or "cleaning^" energy.

A swarf particle of rectilinear shape will be rounded to some extent when it is impacted by the hammer in a hammer mill and this rounding effect will be increased in proportion to the number of times that the swarf particle is impacted. That this rounding does in fact take place is demonstrated when the swarf particles are used as shot. In each cycle of a load of^" shot, some percentage of the swarf particles, catapulted by the machinery- impact a work piece. The impact is analogous to the impact of the hammers on the particles in a hammer mill as described above. Because the spherical pellets of conventional shot are also distorted due to these impacts, it has been observed that, after a certain amount of use as shot, the swarf particles become rounded to the extent that they are difficult to distinguish from used pellets of conventional shot. This is the reason for the assertion that the swarf particles benefit from being impacted by the hammers in the crushers, particularly the final stage crusher 34. In order to take advantage of this effect and to deal, at least to some extent, with the disadvantages outlined above, the process described with reference to Figure 1 is modified. Before being taken to the grading trommel 38, the swarf particles which emerge from the final stage crusher 34 are passed into what is conveniently called a conditioning crusher 80 illustrated in Figure 10. Many of the components of the crusher 1 10 have counterparts in the crusher 80 which are identified in Figure 10 with the same reference numerals as appear in Figure 5 but with a dash (') appended Except as discussed below, it may be taken that these components are essentially identical in construction and operation and it is not considered necessary to describe them in detail.

However, there are certain important points of difference. The crusher 80 composes a hemi- cylindrical lower casing 82 located below a rotary hammer assembly comprising an aπay of radially disposed arms 1 14' each of which caoies at its outer end a hammer 1 16^" In the present case the lower casing is made up of six curved segments 84 of equal angular size, each subtending about 30° at the centre of curvature of the lower casing. The lower casing 82 and the hammers 1 16^" are discussed further below The crusher 80 comprises an upper casing 138' which overlies the hammer assembly and is provided with a first aperture 136^" through which swarf particles from the final stage crusher 34 are fed into the crusher 80 The first aperture is located directly above the hammer assembly

The major part of the lower casing 82 is unperforated. However, one of the segments 84, being the last of the segments traversed by the hammers 1 16^' as

rotate in an anticlockwise sense as viewed in Figure 10. constitutes a god 126^" comp sing perforations 130'. The perforations are of tapered cylindrical form, being of smaller cross sectional size at the inner face 128' of the grid than at the outer face 132'. The diameter of the perforations 130' is 2 mm at the inner face 128' but may be 3 mm or 4 mm (or other suitable size) in circumstances which will be discussed. Each segment 84 shown here is a casting, 6 mm thick and having a 70 cm radius of curvature.

A second aperture is provided in the casing 138' immediately adjacent the segment grid 126'. This aperture is closed by a trapdoor 85 and is discussed further below. The hammers 1 16' have a profile which, viewed in the axial direction, is bell shaped and comprises a wide outer portion 86 which has flat leading and trailing faces 88, 90 and tapers to a narrow portion 92 at the inner end of which there is provided a passage through which a pin 124' passes with clearance for attaching the hammer 1 16' to the outer end of the arm 1 14'. The outer face 94 of the hammer is smooth and rounded.

The hammers 1 16' are free to swing on the arms 114' and are capable of a substantial degree of movement in the radial direction on the arms. The geometry of the arrangement is that, when an arm 114' which carries a hammer 116' is aligned with any of the segments 84 comprising the lower casing, in the extreme outer position of the hammer on the arm, there is clearance (of 2 mm in this example) between the outer face 94 of the hammer and the inner face 128' of the segments 84. The swarf particles 56 discharged from the final stage crusher 34 enter the conditioning crusher through the aperture 136'. Under the action of the hammers 1 16' they are driven (anticlockwise in the drawing) around the shell of the crusher 80 and traverse first the unperforated segments 84 of the lower casing. Before the swarf particles reach the grid 126' there is no route by which any of them can escape. In any case most of the swarf particles are larger than the perforations in the grid 126', so they cannot escape through the grid even when they reach it. The bulk of the swarf particles thus continue to be driven round the crusher 38 as long as the trapdoor 85 is kept closed. In the course of this movement they are thus impacted, on average, many times by the hammers.

Being of engineering steel, the swarf particles are tough and the impacts cause the bulk of the swarf particles in the conditioning crusher to be become progressively more rounded in shape. Figure 4 illustrates the shape of particles 58' which originally had a shape typified by that of the particles 58 but which have been conditioned in the conditioning crusher Their greater roundness is evident The conditioning crusher is not intended or designed to reduce the size of the swarf particles except to the extent of rounding them in shape This is why the faces 88, 90, 94 of the hammers 116' are smooth Some of the smaller swarf particles are doven through the perforations in the god These will also have been rounded and can be earned by a belt conveyor 96 to a storage bin and subsequently passed through the grading trommel

In due course the trapdoor 85 is opened, in the present case by an air cylinder (not shown), and the rounded swarf particles remaining in the conditioning crusher are doven by the hammers out of the second aperture from where they drop onto a belt conveyor 98 and are ultimately passed through the grading trommel 38 in the manner already described The nature of the action of the conditioning crusher is better suited to treating batches rather than a continuous feed of swarf particles By controlling the amount of time a batch of the swarf particles remains m the conditioning crusher, the particles can be rounded to the point where their shape is close to that of conventional cast pellets

The hammer assembly of the conditioning crusher here described rotates at 500-900 rpm By way of illustration, if the hammer assembly composes four hammers as shown and is rotating at 700 rpm, the swarf particles in a batch which is held in the crusher for 15 seconds potentially undergo 1225 impacts In a typical shot blasting machine, a batch of shot typically circulates once in two minutes In these circumstances the particles of shot would take 147000 seconds to impact a work piece 1225 times Although this is a somewhat assumptive calculation, it does illustrate the relatively very large number of impacts which the swarf particles undergo in a short time in the crushers, and particularly the conditioning crusher, as compared to when the particles are in use as shot in a shotblaster More than one of the segments 84 adjacent the trapdoor 85 mav incorporate gods and the size of .he perforations m each grid can be selected so that swaif^" particles of different size can be doven through each god

A typical batch advantageously comprises up to 200 kg of swarf particles which were discharged from the end of the final stage crusher An output of 1 -2 tonnes per hour of satisfactorily rounded swarf particles is achieved using a 350

motor to drive the hammer assembly in the crusher 80

Because it is not considered technically essential that the swarf particles discharged from the crusher 34 need to be conditioned, they may be passed first through the grading trommel. In that case only the oversize particles 58 might then need to be conditioned and subsequently recycled through the grading trommel.

The swarf particles are considerably tougher than the brittle high carbon steel of which conventional cast steel pellets are made. Consequently the cast pellets are more likely than the swarf particles to be broken up impacts. This is another reason why the appearance of the swarf particles becomes close to that of the cast pellets in the course of use. One advantage of the shot prepared by the methods of the present invention is that it produces little dust in use. A second advantage is that it is cheap to produce. The raw material (swarf) is cheap and the plant for processing it is cheaper than the plant for producing cast steel shot. Furthermore, the process for producing cast steel shot involves the use of a steel melting furnace which consumes a considerable quantity of energy and generates a considerable amount of pollutants including dust, slag and greenhouse gases. The process of the present invention is more environmentally friendly.

It is not intended that recognised mechanical equivalents of and/or modifications of and/or improvements to any matter described and/or illustrated herein should be excluded from the scope of a patent granted in pursuance of any application of which this specification forms a part or which claims the priority thereof or that the scope of any such patent should be limited by such matter further than is necessary to distinguish the invention claimed in such patent from the poor art

Claims

I . A method of recycling swarf CHARACTERISED IN THAT raw swarf is treated to produce a mass of swarf comprising swarf particles [56, 58, 58'] which are of selected size and which are suitable for use as shot in a shot blasting operation. 2. A method according to claim 1 CHARACTERISED IN THAT the raw swarf comprises swarf chips at least some of which are reduced in size to produce the mass of swarf particles.

3. A method according to claim 2 CHARACTERISED IN THAT at least some of the swarf chips are reduced in size to produce a mass of swarf pieces and at least some of the swarf pieces are reduced in size to produce the mass of swarf particles.

4. A method according to claim 2 or claim 3 CHARACTERISED IN THAT the swarf chips are reduced in size by passing the swarf through at least one hammer mill [18, 1 10].

5. A method according to claim 3, CHARACTERISED IN THAT the swarf pieces are reduced in size by passing the swarf through at least one hammer mill [20, 34, 110, 80]. 6- A method according to any one of claims 1 to 5, CHARACTERISED IN THAT the swarf is cleaned to remove unwanted matter.

7. A method according to claim 6, CHARACTERISED IN THAT the swarf is heated to remove unwanted liquid matter.

8. A method according to claim 1 or claim 2, CHARACTERISED IN THAT at least some of the swarf particles [58] are subjected to a shaping operation in which the swarf particles are rounded

9. A method according to any one of claims 3 to 7, CHARACTERISED IN THAT at least some of the swarf pieces [58] are subjected to a shaping operation in which the swarf pieces are rounded. 10. A method according to any one of claims 1 to 9, CHARACTERISED IN THAT the swarf is comprised substantially of engineering steel.

I I . A method of recycling swarf according to any one of claims 1 to 10, CHARACTERISED IN including the step of using the swarf particles as shot in a shot blasting operation.

12. Shot CHARACTERISED IN HAVING BEEN produced by a method according to any one of claims 1 to 10.