Steel grit versus slag – a brief comparison
27.02.2017

The ship tanks - ballast, fuel, cargo and voids - have been historically blasted while using a variety of mineral abrasive types.

Since many of these tanks are usually contaminated with oil, salt or other rusting pollutants, using a non-recovery abrasive might have a negative impact on the environment.

Furthermore, through blasting operation contaminants such as oil, grease and salts cannot be cleaned from the metal surface. It is therefore necessary, to thoroughly wash the tanks prior to blasting them. This operation can be done with water and cleaning agents able to remove the above mentioned substances.

The removal of surface contaminants prior to blast cleaning overcomes the abrasives’ contamination, which is one of the major obstacles to using recyclable media while blasting in tanks.

On the other hand, when the ships are cleaned, the recyclable abrasives will not pick up oils, grease, or salts. This will allow reusing them during series of blasting cycles.

Opting for slag for blasting purposes presents a set of disadvantages related to operational safety, productivity and environmental protection. Part of these disadvantages will be mentioned below:

• Breakdown - Mineral abrasives and mineral slags in particular tend to disintegrate on impact, generating large volumes of airborne dust. The dust creates a health hazard for blasters and others in the work area, causes poor

visibility and reduces the blaster’s productivity.

• Material Handling - More abrasive is required, both in volume and in pounds per square foot, to clean with mineral abrasives compared to steel abrasives. This translates into greater handling costs and more complex logistics to continually bring in new material and take away used material.
• Cleaning Efficiency - A recent innovation to abrasive blast cleaning has been the use of higher nozzle pressures, in the range of 120 - 150 psi, resulting in productivity increases of 125- 150% when using steel abrasives. Using mineral abrasives at these elevated nozzle pressures does not dramatically increase productivity because most of the added energy is expended in particle breakdown and increased abrasive consumption.
• Environment - Disposal of the large volumes of potentially hazardous mineral abrasives typically generated by most shipyards is becoming increasingly more difficult and costly, and creates an environmental problem.

Steel abrasives have two major advantages over alternative abrasives: durability and density. The importance of durability and density in terms of abrasive recyclability is discussed below. 

To be truly recyclable, an abrasive must be durable, that is, an abrasive mix must be stable enough from the geometrical point of view so that it will not suffer major modifications during the blasting process. 

The data above shows that after a single impact mineral abrasive loses almost 50% of its original size compared to steel abrasive, which lost only 1%. Steel abrasives are extremely durable and can be reused hundreds of times without losing size or cleaning ability. Mineral abrasives, on the other hand, if recycled would, after one or two recycles, be too fine and dusty to bean effective abrasive.

The second major attribute of steel abrasive, density, is the key to recyclability. Steel has a specific gravity of 7.4 compared to mineral abrasives, which are typically 2.5 to 3.5. Therefore, steel lends itself to simple air classification as a means to remove lighter paint chips, fines and dust from used abrasive. The most common method of air classification is the “air wash” which passes a controlled flow of air through a measured flow of abrasive. The air-flow is set such that it will sweep out the paint chips, fines and dust leaving a cleaned steel abrasive product for reuse. Mineral abrasives, on the other hand, have close to the same density as the paint chips, fines and dust contaminating the abrasive. Air washing mineral abrasive is not practical since it will remove almost all the abrasive along with the contaminants. 

Maintaining abrasive cleanliness is the key to successful abrasive recycling. Periodic sampling and checking of the recycled abrasive for contaminants such as salt, oil or heavy metals should be incorporated into the production schedule. The Steel Structures Painting Council has a draft specification for recycled steel abrasives. This proposed specification outlines the specific physical and chemical tests that should be run to assure abrasive cleanliness.

Steel abrasives are the ideal recyclable abrasive product based on durability and density. However, there are some precautions to be considered when using steel abrasives. Of primary importance is keeping steel abrasive dry. Steel will rust if allowed to sit in water and with time this rusting can cause the abrasive particles to form lumps, which can plug the system. A small amount of moisture is no problem if the abrasive is kept moving and the moisture can then be removed by air dryers in the blasting process. 

To summarize, steel abrasives, because of their durability, can be recycled hundreds of times before the particles become too fine for reuse. With recycling, many of the logistical problems encountered with mineral abrasives, such as daily receiving and disposing of truckloads of abrasive are eliminated. Higher nozzle pressures can be used with steel abrasive thus increasing productivity by 125 - 150% while reducing disposal costs by 99%. Steel abrasives offer increased productivity, long life and excellent recyclability compared to mineral abrasives. To realize these advantages, abrasive cleanliness must be maintained along with moisture controls on all compressed air sources.