The purpose of this overview is to identify the major components of effective equipment for recycling garnet abrasives. Also covered are several rules that are critical to understand when recycling garnet (or any non-ferrous abrasive). Lastly, how to monitor the effectiveness of a recycling operation is explained. Following these guidelines can help ensure that each blast with BARTON garnet abrasives is as productive as the first.
Key Components of Effective Recycling Equipment
The most vital component of any recycling system is the method used to separate and classify the used abrasive. Experience has shown that systems that utilize a rotary drum (or screen) followed by a gravity-fed air wash are very effective for recycling garnet. A vibratory screen below the air wash has proven useful for removing additional contaminants and improving the level of cleanliness of the recycled garnet. Other major components of a typical recycling system include an inlet hopper, bucket elevator/auger combination, integrated storage hopper (to store recycled abrasive) and a dust collector.
Depending on the application, a level of cleanliness of 90-95% should be possible. Several manufacturers produce recycling systems that are durable and effective at recycling BARTON garnet abrasives. A recycling unit designed specifically for a ferrous abrasive such as steel grit may not be effective at recycling garnet. This is due to the difference between the specific gravity of garnet and the specific gravity of steel grit.
For a field recycling operation, a collection tank (for storing the abrasive to be recycled), a virgin abrasive storage hopper (for virgin make-up abrasive) and a waste bin/drum are also needed. Two other important components of any blast project are the method of abrasive collection (automated, vacuum or manual) and the way in which the blast pot is charged with recycled abrasive.
Overview — Recycling Garnet Efficiently
The goal of recycling is to clean the blasted abrasive so it can be used again productively. Ensuring that the recycled abrasive (known as the “working mix”) is clean and contains abrasive particles with the appropriate size range involves two basic operations: the removal of oversized and undersized contaminants, and the addition of virgin (make-up) abrasive. In order to accomplish this efficiently, both the blasted and virgin make-up garnet must be kept dry.
Decontamination of the blasted (used) abrasive is accomplished by mechanical separation and air-wash separation. A rotary drum removes the large contaminants such as large paint chips, rust, welding rods and cigarette butts. An air-wash separator unit, working in conjunction with a dust collector, removes unwanted fine abrasive particles (“fines”).
The most critical element of the recycling process is the creation of an effective “working mix” that comprises a range of small, medium and large abrasive particles. Given that a portion of the garnet breaks down each time it is blasted, a productive working mix should be created by adding virgin abrasive to the cleaned abrasive at a rate equal to the rate of attrition (i.e., breakdown rate). These additions should be made at regular intervals and be based on the amount of fines extracted by the separation unit, daily visual inspections of the recycled abrasive and periodic sieve analyses.
Adjusting Recycling Equipment
Most systems utilize some form of air-wash separator because they can be precisely adjusted to extract fines from the dirty abrasive. It is important to use a system with a properly designed air wash. A well-designed air-wash separator works much like a vacuum cleaner. A thin curtain of dirty abrasive slips under a swinging baffle and then passes through the air wash. The air flow (or suction) created by the dust collector pulls fines and airborne dust into the dust collector. Heavier fines are scalped off by a lower separator lip.
It is critical to remove as many fines as possible during each recycling operation. Excess fines can reduce productivity, create unwanted dust during blasting, reduce blaster visibility, reduce profile, and supplant usable abrasive particles that should be passing through the blast nozzle.
An even flow of abrasive through the air-wash separator should be maintained. A partial or gapped abrasive curtain causes usable abrasive particles to be separated out when the abrasive flow/curtain is sparse and leaves contaminants in the abrasive in places where the flow/curtain is heavy. The two-thirds adjustable baffle and the swinging baffle determine the distribution and thickness of the abrasive curtain as it passes through the air wash. The adjustment of these baffles must be calibrated for the specific grade and weight of abrasive being recycled.
The four typical adjustment areas of the air-wash separator are: the amount of air-wash “gap” between the lower separator lip and swinging baffle, the gap below the two-thirds adjustable baffle, the amount of counter weight on the swinging baffle, and the volume of air flow through the air-wash “gap.” When making adjustments, it is recommended that only one setting be changed at a time.
Excessive air flow within the air-wash separator can remove large, useful abrasive particles. Too little air flow can result in too many fines being left in the cleaned abrasive. In some units the dust collector also extracts dust from the bucket elevator and rotary drum housing.
The volume of air flow across the air-wash separator is controlled by adjusting the gate in the air suction duct that leads from the dust collector to the air-wash separator housing. To leave more fines in the cleaned garnet, the gate should be closed slightly. To remove more fines, the gate should be opened slightly. A minor adjustment of this opening can have a big effect. A 1/4″-1/2″ adjustment is considered a major adjustment.
The dust-collector slide gate should be set so that enough air flow is drawn through the abrasive curtain to remove most of the fines and other contaminants. The air flow or suction created by the dust collector will cause the abrasive curtain to bend toward the lower separator lip. If adjusted properly, fines and dust are pulled over the lower separator lip and gravity fed to a waste hopper. Clean abrasive should hit near the top of the lower separator lip and drop into a collection hopper for reuse.
The “Working Mix”
A properly balanced particle size distribution in the abrasive working mix is the key to effective and efficient blast cleaning. A good working mix contains a well-balanced distribution of large, medium and small abrasive particles. Large particles perform the major task of loosening the thick, heavy contaminants. Small particles provide the coverage necessary for the efficient removal of lighter coatings, mill scale, and rust. Smaller particles also perform the important task of thoroughly cleaning and scouring pits.
Establishing a suitable range of particles in the working mix is application-specific and is based primarily on the profile requirement and the coating or contaminant to be removed. Typically, garnet particles smaller than 80-90 mesh should be extracted from the working mix when removing coatings over 20 mils or medium rust. For blast applications removing light rust, mill scale, primers and thin coatings, the working mix can be broadened to include 100-mesh or even 120-mesh garnet particles.
Each abrasive particle type, size and shape has its own impact life cycle (i.e., breakdown rate). Each time the abrasive is blasted, a portion of the particles break down. The amount of air pressure used to blast naturally affects the breakdown rate, but the relationship is not linear. Generally, one can expect a 20-25% breakdown rate at 90-110 psi at the nozzle with BARTON 30/60 Plus. As a rule, add back 25% make-up abrasive with each recycle unless a shallower profile or a finer abrasive is desired.
Virgin abrasive should be added into the working mix at approximately the same quantity as what is pulled out by the recycling system. As an example, for every 20 tons of garnet abrasive blasted and collected, approximately three to four tons will breakdown to either dust or non-usable fine particles. This make-up abrasive can be added at the bottom of the bucket elevator or at the recycled abrasive outlet to ensure proper mixing of the previously blasted and the virgin abrasive components. Not adding new abrasive decreases the percentage of coarser size particles in the operating mix and leads to slower cutting, increased dust and reduced profile.
The recycled abrasive should be checked to ensure that the separation unit is removing both oversized coarse contaminants (typically 20 mesh or larger) and the fines (typically 80-90 mesh or smaller). The abrasive discharged from the air-wash separator (into the trash/dust bin) should also be checked to determine if usable abrasive is being improperly withdrawn from the working mix. Abrasive samples should be taken directly from the air wash as the abrasive falls past the swinging baffle and from the air-wash discharge tube, rather than from a stationary source.
The most accurate way to determine the exact distribution of the working mix of the abrasive particles is to conduct a sieve analysis. Complete instructions and procedures on the use of the test sieves are contained in ASTM STP 447, Manual on Test Sieving Methods. For a detailed sieve analysis, use #20, #30, #40, #50, #60, #70, #80, #90, #100 and #120 mesh USA Standard Sieve screens. A full set of screens is invaluable for monitoring the particle size distribution of the working mix.
A short-cut method used to reduce the complexity of conducting a complete sieve analysis is to select the “critical” screen for the blasting application where recycled garnet is being used. For example, to check for excessive fines in the working mix, use a single #80 or #100 mesh screen. To check for too much usable garnet being extracted by the air-wash separator, use a single #60 mesh screen. This will give a relative indication of how well the recycling process is being managed and point out any potential problems.
Routine visual checks at least two or three times per shift should be made by qualified personnel at the discharge tubes leading from the air-wash separator and rotary drum. Check for usable abrasive in the discharge and fines in the recycled material. Check for a uniform and full abrasive curtain in the air wash. Clean out all abrasive from the trough (if there is one) below the rotary drum, directly before the swinging baffle, each shift. If it is not cleaned out regularly, dust builds up as abrasive stacks up before the swinging baffle. This causes gaps in the abrasive curtain and negatively affects the efficiency of the air wash.
BARTON has practical experience with recycling in blast rooms and on field projects. We are familiar with a variety of designs for recycling equipment, as well as abrasive handling and flow issues encountered during recycling. Our goal is to provide our customers with unparalleled technical support for both blasting and recycling garnet abrasives.
Contact BARTON or a qualified distributor familiar with recycling garnet prior to purchasing any equipment. Arrange for a field demonstration and be prepared to institute a training program for personnel who will operate the equipment.