Fly Ash Recycling & Processing

A patented process (U.S. Patents 5,047,145, 5,227,047 and 6,027,551) has been developed by the Institute of Materials Processing at Michigan Technological University, Houghton, Michigan, for the beneficiation of coal fly ash. Froth flotation is used to remove carbon and enhance the fly ash. A private company has licensed the patent rights for commercialization to produce enhanced pozzolan fly ash and marketable carbon.

As Received Flyash
Scanning Electron Microscope (SEM) at 1000X
Cenospheres removed during benefication. Hollow spheres
with A1/Si ratios varied from 1:1 to 1:2. Scanning Electron
Microscope (SEM) photograh at 100X
Large silicate pleurosphere
Scanning Electron Microsope (SEM) at 200X
Process
The wet process for beneficiation of a fly ash byproduct has the following selected steps:
1) forming a slurry mixture of fly ash material and a liquid;
2) gravitationally removing cenospheres from the slurry;
3) separating the unburned carbon from the remaining slurry components by froth flotation; and
4) collecting the remaining fraction of fly ash. The products are dewatered and dried for shipment to respective markets.
Equipment Systems
The flotation plant consists of fly ash raw feed silos, wet conditioning and slurrying system, primary and secondary flotation cells, dewatering equipment, drying equipment, conveying systems, finished product storage silos, and material handling equipment.
Normally the liquid portion of this process is a closed loop system. A small amount of make-up water is required to replace water evaporated in the drying process. The system is adjustable to accommodate and match up with certain existing plant equipment. This can lead to substantial cost savings of purchased equipment and operating cost.
Plants generally can be designed to accommodate any annual fly ash production rate, from low volume to very high volume. Plants can be designed with production rates as low as 5 tons per hour to as high as 80-100 tons per hour.
System Advantages
Froth flotation is a well understood and proven technology that has been used by the minerals processing industry for over eighty years. It is the most important and versatile mineral processing technique and is used to treat a wide range of metallic ores, industrial minerals, and coal.
The MTU process is a system that will consistently and routinely reduce carbon in both low and very high carbon fly ashes down to a level of 1% or less in an effective and efficient manner. The process can effectively handle variable LOI ash as well.
The MTU system produces the optimum product. This system, even at high hourly throughput can produce a beneficiated pozzolan with 1% or less carbon at a 90% or greater recovery rate. Carbon is produced at a 80% purity level, which may be further processed if desired.
The MTU system also may be designed to recover cenospheres and magnetite, should economics and available quantity justify.
Total operating costs, including capital costs, are estimated between $6.00 and $12.00 per ton. The range is a result of many variables, such as: the required plant size, production rates, climate, production days per year, etc. Also, where existing equipment, such as silos for storage of finished product and load out facilities are available, capital cost could be lower, reducing overall operating cost.
Product Advantages
The patented MTU process removes a mineral called Basanite (CaSO4) from the surface of the fly ash. Basanite is a gypsum-like material that has a retarding effect on cement in concrete mixtures. The removal via wet processing improves early set and strength in concrete.
The patented MTU process produces a consistent enhanced pozzolan product ready mix producers desire. A carbon content of 1.0% or less has virtually no effect on entrained air in concrete. A carbon content of approximately 3%, for example, has a dramatic effect on air entrained concrete.
The ash produced by the patented MTU process can be utilized as is for higher value applications because of its high quality and consistency. The ash can be further processed for even higher value uses. Some of these include uses for industrial fillers in plastics, rubber, and specialty applications.
The carbon product produced by the MTU process has a purity of 80% or greater with multiple application potential that clearly sets it aside from other processes.
The patented MTU froth flotation process is equally effective in removing all of the different types of carbons in the ash. Recent data indicate that some dry separation processes remove more graphitic carbon, which is less adsorbent than carbon produced by our process.
The byproduct carbon produced by the patented MTU process has a value based on a number of market areas, including at the least, as a high BTU fuel. Current market research indicate the carbon produced by the MTU process should be able to be used in traditional markets, such as purification and activated carbon.
The high purity carbon has the potential to remove mercury and other heavy metals from gas streams at coal-fired plants. This carbon may also be used to treat disposal water from power plants, including discharge effluents.
Environmental Advantages
Another wet process uses kerosene for conditioning. Kerosene use is expensive, as the demand per ton is high, and it leaves an odor in both the enhanced low carbon pozzolan and the carbon. The removal of water laden with small quantities of kerosene also poses environmental problems in the drying phase of the process. The patented MTU process uses low doses of conditioning agents which are very effective, efficient, and most importantly, environmentally friendly.
Construction Requirement
Construction of a carbon removal froth flotation facility can vary depending upon the utilization of existing equipment. Often it is practical to use the existing storage silos and conveying systems for the raw feed system into the wet process. Existing facilities may also allow storage for the finished products. Electrical power and fuel, natural gas or propane, are the two major energies required.
Depending on the size and layout of the existing plant, a plant footprint of one half acre to approximately three acres are required to accommodate the carbon removal plant. The carbon removal plant is enclosed to allow for year round operation.
A typical plant will require from eight to twelve months to complete, after siting requirements are finalized.