Lead-Containing Abrasive Blasting in a Confined Tank
An enclosed 200 m³ steel tank is being abrasive-blasted and coated. The old coating registers 8 % lead by XRF. Blasting uses silica sand. A blower delivers 3,000 m³/h to the tank; the exhaust discharges 6 m from a warehouse air intake. The confined-space permit records O₂ 20.9 % at 08:00; the combustible gas meter carries a calibration sticker dated 14 months ago and its bump-test log is empty. Two blasters work 45-minute shifts in air-fed hoods. The breathing-air compressor intake sits beside the diesel generator exhaust. Spent abrasive is being bagged and sent to landfill as general waste. A neighbouring site has complained about dust.
1. Prioritise the issues on this job and state the immediate actions.
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Stop work, in this order of urgency:
- Breathing air. A compressor intake beside a diesel exhaust can deliver carbon monoxide directly to the hoods - a lethal, odourless exposure, and the highest immediate risk on the job. Shut down or relocate the intake, test the air supply against a breathing-air standard, and add CO monitoring at the point of use.
- Gas detection. A meter 14 months out of calibration with no bump-test record is not a valid instrument. The 08:00 reading of 20.9 % O₂ was taken with a meter whose reliability cannot be demonstrated. Replace it, bump-test before entry, and re-test the space.
- Lead and silica exposure. Blasting an 8 % lead coating releases lead dust and, with silica sand, respirable crystalline silica. Air-fed hoods do not remove the need for exposure assessment, hygiene facilities, change rooms and regulated-waste handling. Silica sand is prohibited as a blasting abrasive in some jurisdictions and restricted in others; confirm the local position and substitute a less hazardous media where required.
- Dust migration. Exhaust 6 m from an air intake, with a neighbour already complaining, is a visible emissions and nuisance problem that may also be an offence.
Resume only when each is corrected and documented.
2. How should the spent abrasive and blasting debris be characterised and managed?
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Not as general waste. The debris is a mixture of abrasive and a coating that is 8 % lead. Blasting concentrates the coating fraction in the fines and dust, so the lead content of the waste can be far higher than the 8 % in the intact film, and the total lead concentration alone is not the regulatory test in most jurisdictions - leachability is.
Characterise before disposal:
- Sample the spent abrasive and dust representatively; the fines fraction is where the metals sit, so a bulk grab sample under-represents the hazard.
- Run the leach test required locally (the toxicity characteristic leaching procedure is the common one) for lead and any other metals present in the coating system, and compare against the applicable limit.
- Hold the waste in labelled, covered containers pending the result - not open bags on site.
Manage according to the result: a leachate exceedance makes it a hazardous or special waste, with permitted transport, manifesting and a licensed disposal route, plus records kept for the required retention period. Even if it passes, keep the characterisation data and disposal records: the 8 % lead finding and the neighbour complaint mean this job is likely to be scrutinised, and undocumented waste decisions are the first thing an inspector asks for.
3. Evaluate the ventilation and air-supply arrangement quantitatively and qualitatively. What would you require before re-entry?
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Quantitatively, 3,000 m³/h into a 200 m³ tank gives about 15 air changes per hour. That is on the low side for abrasive blasting in an enclosure, where visibility control and dust removal typically call for more, and it is the wrong metric on its own: the useful question is airflow at the face and whether the space is held under negative pressure so that dust leaves rather than escapes into the work area. Measure capture velocity at the blasting point, not just blower output on a plate.
Qualitatively, the arrangement has three faults:
- Discharge location. Six metres from a warehouse air intake risks re-entraining lead-bearing dust and is very likely to be the source of the neighbour's complaint. Relocate or extend the stack, and confirm no intake is downwind.
- Breathing-air source. A diesel generator exhaust adjacent to the compressor intake is an acute CO hazard independent of tank ventilation.
- Instrumentation. Calibration and bump-test records must exist before the meter is relied on for entry decisions.
Before re-entry: verified breathing-air quality, calibrated and bump-tested gas detection, documented ventilation performance with the space under negative pressure, exposure monitoring for lead and silica, and a permit that reflects all of it with a competent attendant outside the space.