A project where we acted as consultants, was the comparison of cryogenic separation with other forms of hazardous waste management technologies for the treatment of surface coatings and adhesives.
Cryogenic separation is the separation of materials at a temperature that is below the freezing point of both materials. At this temperature both materials become brittle and consequently can easily be separated by impact. To enable the materials to reach such low temperatures they are either immersed in a bath or sprayed with liquid nitrogen. Nitrogen is non-toxic, relatively non-reactive and boils at -195.8oC (77.2K).
In the USA, cryogenic separation technology has been used since the early 1990’s in the waste tyre industry for the production of rubber crumb. The main advantage of cryogenically frozen rubber is that it pulverises far more easily than rubber at ambient temperatures and therefore requires less power to do so. It also creates less wear and tear on machinery and maintenance costs are usually significantly reduced. The main disadvantage is the cost of the liquid nitrogen system.
Cryogenic grinding is widely used for the reduction in size of particles. A small particle size is desirable when mixing materials, as cooling to cryogenic temperatures makes the materials brittle and consequently easier to break into small particles. Such materials include Adhesives and Waxes, Explosives, Spices, Thermoplastics (nylon, PVC, polyethylene and polypropylene) and Thermosets (natural and synthetic rubber).
The tempering of metals and materials in the cryogenic temperature range developed as a result of the US space program. It was realised that materials that had been exposed to the cold temperatures of outer space, exhibited an improvement in their properties (e.g. tensile strength, toughness, stability and reduction in internal stresses). Consequently Cryogenic Tempering is the process of tempering parts by exposure to temperatures below -150oC, followed by controlled warming to 260oC.
Cryogenic separation has been used to produce quality PVC powder regrind from fabric-backed PVC materials. The presized material is first frozen by liquid nitrogen cooling. It is then passed to a mill where it is ground down to a powder and separated from the fabric which remains fairly flexible and resists the grinding operation. Screening then separates the ground PVC and fabric. Between 70 to 85% of the PVC component is recovered by this system .
In 1981 Ford were using cryogenics separation to reclaim chrome-plated ABS car grilles. The process involved presized scrap being vacuum conveyed into an enclosed liquid nitrogen cooled mill. The ABS was removed and ground into a fine powder, whilst the metal was removed by magnetic separation and conveying equipment.
It is apparent that the many potential techniques that could be employed have limitations, not only in terms of operational efficiency, initial capital cost and operational costs, but environmental impact. As a variety of plants do exist to deal with this complex disposal issue, they require evaluating to assess their true potential and allow bench marking against a standard in order to identify the real costs and issues. This would then indicate potential solutions that would be viable in both financial and environmental terms.
Consequently in this project we:
Examined the use of cryogenics as a waste disposal tool for hazardous substances
Compared this technique with those of landfill, incineration, pyrolysis and chemical separation.
Investigated the potential for energy recovery using the waste as a fuel and recycling the containers
Developed for each potential option a model showing the life cycle costs, energy balance & efficiency
Visited sites identified within the study to obtain operating information, and comparisons of actual performance with predicted performance.
Analysed processes in order to identify key operations difficulties and suggest possible methods of improvement.