Abrasive recovery

Causes and consequences
Under the effect of shot blasting operations:
1) The contaminated agents removed from the parts treated such as sand, calamine, rust, paint, metallic dusts or not, etc, are carried into recovery device and mix with the abrasive. If they are not eliminated as soon as they are recovered, they cause the very fast accentuation of the wear of the installation parts.
2) The abrasive wears out or break because of the impacts repetition and this modifies the qualities of the operating mix. The installation output decreases then if the non usable grains re not eliminated from the installation and the result of the treatment obtained declines. It is thus necessary to maintain the abrasive in a constant property state and the operating mix in the granulometry defined for the treatment in order to ensure the optimal output of the installation.
The function of the recycling units is thus to treat the abrasive, before stocking it, with a view to feed the projection machine for a new use.

Composition
The recycling unit is made of an elevator unit for the abrasive which feeds a connection in series in different orders and selections:
- a sifter to evacuate the foreign bodies,
- a purifier which removes the dust and the scraps,
- a shapes selector (spherical grains) which removes the grains not conform,
- a machine to grade which selects the granulometries,
- a magnetic separator which removes or separates the types of particles or grains,
- a storage buffer reserve of reusable abrasive.
All these elements are adapted to the nature and the outflow of the abrasive projected. Their design and their position depend essentially on the bringing up system which can be of 2 versions:

- the units with mechanical bringing up through an elevator,
- the units with dynamic bringing up through an aspirator (see types machinery).

Mechanical recycling through an elevator

Principle
The abrasive is collected through gravity in a hopper situated at the bottom of a belt elevator fitted out with buckets. The set belt/ buckets, driven by a rotating pulley, brings up and throws the abrasive in a buffer capacity or directly in the abrasive purifier.

Dynamic deviation purifier
A directional draught crosses the polluted abrasive abandoned in free fall and eliminates through deviation the particles with a dynamic resistance superior to the one of the abrasive, or with an inferior density. The mix to purify falls under the form of a curtain with a low thickness in a reserve of purified abrasive. The draught generated by the suction of a filter crosses this curtain with an adjustable speed and carries the impurities in a reduction of pressure caisson.
The only fine and light dusts are sucked up towards the filter.
The intermediary fringe can be recycled towards the shot circuit.
The heavy dusts are evacuated towards a waste barrel.
This kind of purifier is widely used on free shot equipments (compressed air) and the wheels machines.

Magnetic separation purifier
It is used on equipments dealing with not metallic parts, or metallic amagnetic, such as light alloys or stainless steel. When a not metallic abrasive to epurate contains a quantity of magnetic impurities resulting from shotblasting operations on the floor gratings, the part carriage or any other accessory of the cabin, a magnetic drum makes it possible to separate these undesirable particles of the abrasive.
The magnetic drums are made of an amagnetic cylindrical casing turning around a fixed magnetic sector. The ironmagnetic particles submitted to the attraction of the sector are plated and retained against the turning casing which drives them into its rotation, making them going out of the magnetic sector in order to release them in a waste barrel. The abrasive thus purified falls into a storage reserve. This system can equally be used for selecting one or the other of the separated products.

Nota:
- residual pollution, as low as it is, requires a way through a dynamic deviation purifier, immediately after the magnetic drum.
- the magnetic separators are also used on bi-products installations, (shot and corundum) in order to separate the 2 kinds of abrasive before being stored in their respective reserve.

Trommel and vibratory separator
When the quantity and the size of the impurities to eliminate or the foreign bodies is important, it is necessary to add a draining system located before the separators. It can be either a trommel (rotative separator) or a tilted vibratory separator, both of them with a continuous ejection.

Classification of the abrasive
When the specific conditions of the treatment require:
- a rigorous granulometry of the abrasive, for example to obtain the creation of a precise roughness,
- a guaranteed shape of the shot, for example with shot peening where the broken grains have to be eliminated, additional apparels are set on the recycling circuit, such as:
- vibratory separators or screens,
- purifiers of spherical shots,
- separators of broken grains.

Cyclonal recycling through an aspirator

Principle
The abrasive is sucked up through a pipe joined to a hole situated at the end of the recovery system on the floor. Then it is goes up to a set of cyclonal separator/ purifier, working under the effect of the strong depression generated by a filter or a vacuum pump.

Cyclonal separator with unique rejection
The mix to purify (A) is introduced tangentially in the top of a closed cylinder, fitted with an acute cone in the low part. The abrasive carried along by the draught in a swirl is sticked along the partition wall thanks to the centrifugal force. The finer and lighter dust or impurities (B) are not sticked through the centrifugal effect and are sucked up by the main shaft in order to be evacuated (G) towards the filter.
An additional shaft fitted with an adjustable cone is set in applications with microbeads requiring a shape grading.
Once the rotary speed is reduced through a friction against the partition wall, the fragments which offer more dynamic resistance than the beads are thus sucked up (C) and evacuated through the main shaft towards the filter (G). Adjustable atmospheric air entrances (D) make it possible to refine the adjustments. The refined abrasive (E) flows out by gravity in a storage reserve.

Cyclonal separator with double rejection
The working principle is identical to the previous one, but with a separation at the level of the rejections in order to collect the broken abrasive. The main shaft collects and rejects the dust and impurities without abrasive fragments towards the main filter.
The additional shaft, adjustable in hight (H), collects and rejects the abrasives fragments without dust (I) towards an additionl filter.
These double rejection separators are always used, each time the quality of the treatment requires a shape guarantee for the abrasive, (i.e. in shot peening or fine aspect glass blasting) and the unusable abrasive can be recovered for other useful aims.

Nota:
A valve of type lock isolates the separator and feeds the abrasive reserve. Knowing the purifying system is in constant depression, the sifter which retains the foreign bodies is set up between the separator and the reserve.