Bituminous coal beneficiation
An ever increasing majority of the bituminous coal (and anthracite) production is beneficiated through specific gravity and froth based coal preparation. From 1985 through 2000, the number of coal mines in the United States dropped from in excess of 4 000 to 800 with this number now starting to increase. There was a corresponding drop in the number of coal preparation plants from almost 500 to 212 while there has been a consistent increase in the percentage and tonnage of the recovered bituminous coal that has been beneficiated (Fig. 4). This apparent anomaly has been due to:
- The introduction of more capital intensive/less labor intensive automated mining equipment that has dictated 24 hour per day, seven day per week operations (previously not permitted in the Collective Bargaining Agreements with the Unions).
- The corresponding closing of smaller, less automated mines.
- A reduction in eastern Untied States surface mines due to more stringent reclamation laws.
- An emphasis on the production of a lower sulfur product.
- Simplification and standardization of coal preparation technologies which has increased plant availability.
The best example of the above change is that in the 1970’s and 1980’s that a large bituminous deep mine produced 1 000 000 tonnes per year with 700 people while a current large deep mine produces 5 000 000 tonnes per year with 225 employees. Comparatively few new coal preparation plants were built between 1985 and 2 000 although essentially every retained coal preparation plant was significantly upgraded. From 2000 through 2005 there has been an increase in the number of coal preparation plants with there currently being 265 operating coal preparation plants in the United States.
Due to blending and other practices, it is essentially impossible to determine how much of the US bituminous coal production is beneficiated. The operating US coal preparation plants have an inplace feed capacity of approximately 150 000 tonnes per hour. If these plants were on line 75% of the time (combination of availability and scheduled operating time) they could process 986 million tonnes of raw coal per year. Based on the current average 57% recovery, this would equate to 562 million tonnes per year of clean coal (the majority of the as marketed bituminous coal). It is noted that from 2000 to 2005 that the average recovery has dropped from 62% to 57%. This reflects a combination of (1) recovery from poorer quality seams (depletion of the higher quality seams), and (2) the more automated mining equipment recovering more “out of seam” dilution.
Coal preparation technologies have become more standardized in the United States emphasizing heavy media vessels, heavy media cyclones, spirals and column flotation. There are some remaining jig and Deister Table plants but essentially every new preparation plant is based on heavy media vessels and cyclones, spirals, and column flotation. The emphasis is on larger plants (> 1 000 tons per hour feed) with larger components (large banana screens, larger diameter heavy media cyclones, column flotation, etc) with increased PLC control. The major issue facing the United States (and Canadian) coal preparation industry is that the lull in preparation plant construction between 1985 and 2000 has resulted in the downsizing or demise of most of the engineering firms specializing in the design and construction of coal preparation plants (McNally, FMC, H&P, Roberts & Schaefer, etc). This has been compounded by the lack of experienced field erection personnel. This has resulted in changes like the detailed engineering of piping which was previously just field installed by experienced personnel.
During the 1970’s there were up to 150 operating thermal coal dryers in North America specifically designed to reduce the surface moisture of beneficiated coal. These thermal coal dryer installations were unique to the United States due to the market specifying a less than 6% surface moisture product (most of the rest of the world operates with a higher moisture content market). Improvements in mechanical dewatering systems have reduced the number of surface moisture thermal coal dryers to approximately 35. In the Untied States these remaining thermal coal dryers are primarily on high Hardgrove Grindability (soft) , frequently metallurgical, type coals where there is an excessive amount of fines. In Canada, thermal coal dryers are used in conjunction with high Hardgrove coals and also to address freezing in train cars in the winter. There have been no additional surface moisture thermal coal dryers idled in North America during the past five years unless the entire mine was idled/shutdown, and one new thermal coal dryer has been built at the Western Canadian Coal Wolverine Mine (furnished by Roberts & Schaefer, Salt Lake City, Utah). It is also noted that BHP Billiton is in the process of starting up two gas recirculation, media bed thermal coal dryers at their Dendrobium Plant in Australia (provided by Heyl & Patterson of Pittsburgh).