1. Field of the Invention
This invention relates to coal flotation, and more particularly to coal froth flotation utilizing gaseous carbon dioxide for production of clean coal concentrates.
2. Description of the Prior Art
Froth flotation is a physicochemical separation process that depends on the attachment of air bubbles to hydrophobic particles. Other (hydrophilic) particles are wetted by the aqueous phase and will not attach to the air bubbles. Thus the separation of coal particles from gangue minerals by froth flotation occurs, for example, as dispersed air bubbles pass through a suspension of coal particles (-28 mesh). The bubble/particle aggregates of coal float to the surface and may be collected as a clean coal concentrate separated thereby from the wetted gangue particles.
Generally this process involves the use of suitable reagents (neutral molecular oils) to enhance the hydrophobic character of coal particles while the gangue mineral particles remain hydrophilic. These neutral molecular oils such as kerosene or fuel oil are called promoters and are used to enhance the attachment of air bubbles at the coal surface by forming a thin oil coating over the coal particles to be floated. Further, a frother is added to establish a stable froth phase to hold the bubble/particle aggregate. In coal flotation, frothers such as methyl isobutyl carbinol, terpinol, cresols, and polyglycols are frequently used. The choice of frother and oil depends on the desired level of selectivity with respect to ash and sulfur.
Although the foregoing known processes are successful for conventional coal flotation, it is more difficult to produce "super-clean" coal by flotation of finely ground coal (-400 mesh). The promoter which is used to increase hydrophobicity inadvertently adsorbs on the gangue minerals, and these gangue minerals subsequently migrate to the concentrate, thus decreasing the quality of the product. Also, reagent consumption is high because fine coal, due to its high surface area, adsorbs significant amounts of promoter and frother. Finally, the rate of fine coal flotation by existing flotation techniques is very slow. As a result, the production of super-clean coal by conventional froth flotation methods has had limited success.
A super-clean coal product is particularly desirable in the production of coal/water fuel. Coal/water fuel contains roughly 70% of the super-clean coal and is stabilized by the addition of various chemical additives so that it can be pumped, stored and used much like oil for which it is intended as a substitute.
In regard to the utilization of carbon dioxide in coal cleaning processes, some work has been done. For example, U.S. Pat. No. 4,522,628 to Savins discloses a method for removing ash from coal using liquid carbon dioxide under pressure in order to fracture and crush coal, not for flotation. In Savins, after comminution at high pressure and elevated temperature with liquid carbon dioxide, conventional flotation is used for coal recovery. Santhanam, in U.S. Pat. No. 4,206,610, also uses liquid carbon dioxide. The carbon dioxide is used in the Santhanam reference as a liquid to replace water as a medium to transport coal from mine to a remote processing plant. However, liquid carbon dioxide processes (transportation and cleaning) have inherent problems which relate to the chemical and physical properties of carbon dioxide (the need to keep it under positive pressure, etc.).
Other references treating coal for various purposes include U.S. Pat. No. 3,998,604 to Hinkley which discloses the use of acids for grinding of coal. Hinkley uses acid treatment (not gas) during grinding. This is a type of leaching reaction. Carbon dioxide is mentioned in passing as a companion to carbonic acid for the sole purpose of grinding and making an acid slurry. Subsequently, the ground coal is floated and whether the coal "floats" or "sinks" is dependent on the flotation reagents, not the gas used.
Steam and gaseous carbon dioxide have also been used in a high temperature coal process. Robinson et al, in U.S. Pat. No. 4,053,285, discloses a steam/carbon dioxide process for chemically treating (using hydrogen) previously prepared coal to lower the sulfur content. At high temperature and pressure, sulfur in the coal reacts with carbon dioxide and steam and is reduced to a sulfide. Flotation is not involved in this prior art reference.
No process is known which utilizes gaseous carbon dioxide to improve the hydrophobic character of the coal surface. That is, no process is known which uses gaseous carbon dioxide as a surface active reagent in the froth flotation cleaning of coal.