STEEL GRIT

CLASSIFICATION SPECIFICATION

When mobile steel grit recycling began in the mid-80's, it was thought that the process was quite simple. Just remove the small particles of steel and dust. We have since learned that there is much more to the process than just taking out the small particles.

7 separate factors must be addressed to maximize the benefits of steel grit recycling to ALL parties.

1. REMOVE oversize particles of metallic and non-metallic materials

This is usually accomplished by mechanical screening using a rotary drum or vibrating screen. It is important to remove large particles of paint, gravel, etc. (non-metallic) and rust (metallic). Because these particles can clog a system, it is important that there be a method of easily removing the oversize materials.

2. REMOVE 99.9% of non-metallic particles (coating dust)

To reduce re-entrainment of dust into the mix, it must be removed during the classification process. Advantage SPS can reduce this to 1/10^th of 1% or 1/10^th of the SSPC specification. The better this is done, the less dust will be evident during blasting. This is very important to get the full benefits of using steel grit.

3. REMOVE "Total lead" down to the 250-500ppm level

Effective removal of non-metallic particles will also usually reduce the "Total Lead" content. Advantage SPS can reduce this to 250-500ppm. This is one half of the 1000ppm level specified by the SSPC.

4. KEEP metallic particles as small as 80-90 mesh size

This is necessary to insure that the "working mix" doesn't become too coarse, causing excessive profile. When this happens, the blasted steel profile can become excessive, requiring primer in excess of manufacturer's recommendations to cover it properly. Applying primer thicker than recommended can cause premature coating failures. Insufficient primer with a high profile can cause premature rust through.

5. SPEED of classification

Advantage SPS classifiers can complete the cleaning at a rate of up to 20 tons per hour. This allows the classification to take place as the grit is being recovered and eliminates the need to "double handle" the grit, significantly reducing exposure by all parties to the lead content. While sustained recovery may not take place at this speed, the system should be designed for peak levels of recovery that can occur when a short recovery hose and high CFM vacuum are used.

6. TOTAL tons of system capacity

The process of blasting and vacuuming represents one (1) cycle. Ideally, you would probably like to have just one cycle per day. This means you need to have enough grit in the system to run all day long before you start recovery. 1.5 cycles means you start vacuuming at lunch and recover about half the grit by the end of blasting. This allows you to run 1.5 times the system capacity in one day.

7. Pressure vessel operation - continuous or intermittant refill

The goal of any properly designed abrasive blasting system should be to maximize the number of hours of ON TIME (blasting) in one day. The key to this is having grit available 100% of the time period during which blasting is taking place. Continuous blasting made possible by double chamber pressure vessels is the best, since it takes the least amount of management. If the system allows the operator to refill at every break period, then a single chamber vessel can provide performance equal to the doulbe chamber unit. The key is to always have grit available for the blasters.

Need specification assistance?

Give us a call and we can help you prepare a specification that will remove the mystery from getting proper surface preparation.

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Copyright 1998, Surface Preparation Systems, Inc.