What is Abrasive Blasting?
Abrasive blasting is mostly related to cleaning painted and unpainted steel. It may also be used with care to prepare concrete and masonry surfaces and to clean and roughen existing coatings for painting. Abrasive techniques used for painting surface preparation are discussed.How Abrasive Blasting Works?
High-velocity abrasive particles driven by air, water, or centrifugal force impact the surface to remove rust, mill scale, and old paint from the surfaces. Abrasive cleaning does not remove oil or grease. If the surface to be abrasive blasted is painted with leaded paint, additional controls must be employed to minimize hazards to workers and the surrounding environment. The degree of cleanliness obtained in abrasive blasting depends on the type of abrasive, the force with which the abrasive particles hits the surface, and the dwell time.Types of Abrasive Blasting
1. Air (Conventional) Abrasive Blasting
In this method, dry abrasive is propelled against the surface to be cleaned so that rust, contaminates, and old paint are removed by the impact of the abrasive particles. Moreover, the surface must be cleaned of blasting residue before painting. This is usually done by blowing clean air across the surfaces. Special care must be taken to ensure that horizontal or other obstructed areas are thoroughly cleaned. Additionally, uncontrolled abrasive blasting is restricted in most locations because of environmental regulations. Consult the local industrial hygiene or environmental office for regulations governing local actions. Finally, procedures for containment of blasting debris are being used for paint removal from industrial and other structures. The amount of debris generated can be reduced by recycling the abrasive. Recycling systems separate the paint waste from the abrasive.
Fig. 1: Schematic Drawing Illustrating Components of Conventional Abrasive Blasting Equipment
Fig. 2: Air abrasive blasting
2. Wet Abrasive Blasting
It is used to control the amount of airborne dust, and there are two general types of wet abrasive blasting. In one, water is injected near the nozzle exit into the stream of abrasive as shown in Fig. 3. In the other, water is added to the abrasive at the control unit upstream of the nozzle and the mixture of air, water, and sand is propelled through the hose to the nozzle. For both types of wet-blasting, the water may contain a corrosion inhibitor. The inhibitors are generally sodium, potassium, or ammonium nitrites, phosphates or dichromates. They must be chosen to be compatible with the primer that will be used. The surface must be rinsed free of spent abrasive after wet blasting. Rinsing can be a problem if the structure contains a large number of ledges formed by upturned angles or horizontal girders since water, abrasives, and debris tend to collect in these areas. The surface must be completely dry before coating. When leaded paint is present, the water and other debris must be contained and disposed properly. This waste may be classified as a hazardous waste under Federal and local regulations, and must be handled properly.
Fig. 3: Schematic Drawing of Cross Section of Typical Water-Injected Wet Abrasive Blasting Nozzle
Fig. 4: Wet abrasive blasting
3. Vacuum Abrasive Blasting
It is also known as dust free or dustless blasting. The blasting machine is equipped with a blast hose and a suction hose, that both run from the blast tool to a control unit. Vacuum blasting systems collect spent abrasives and removed material by means of a vacuum line and shroud surrounding the blasting nozzle. Abrasives are usually recycled. Production is slower than open blasting and may be difficult on irregularly shaped surfaces. Nonetheless, shrouds are available for non-flat surfaces. The amount of debris entering the air and the amount of cleanup is kept to a minimum if the work is done properly for instance the shroud is kept against the surface. This procedure is often used in areas where debris from open air blasting or wet blasting cannot be tolerated.
Fig. 5: Vacuum abrasive blasting
4. Centrifugal Abrasive Blasting
Cleaning by centrifugal blasting is achieved by using machines with motor-driven bladed wheels to hurl abrasives at a high speed against the surface to be cleaned. Frequently, centrifugal abrasive blasting employed to prepare horizontal surfaces for the application of polymer sealants and coatings. Advantages over conventional blasting include savings in time, labor, energy, and abrasive; achieving a cleaner, more uniform surface; and better environmental control. Disadvantages of centrifugal blasting include the difficulty of using it in the field, especially over uneven surfaces, although portable systems have been developed for cleaning structures such as ship hulls and storage tanks. Robots may be used to guide the equipment. In many cases, the abrasive used is reclaimed and used again.
Fig. 6: Centrifugal abrasive blasting
5. Other Blasting Methods
Apart from the above techniques, there are number of emerging techniques and equipment which include:- Sodium Bicarbonate Blasting
- Blast Cleaning with Reusable Sponge Abrasives
- Carbon Dioxide (Dry Ice) Blast Cleaning.