Feed-control, mineral processing

The ore is ordinarily ground to pass through a ca 1.2-mm (14-mesh) screen, mixed with 8—10 wt % NaCl and other reactants that may be needed, and roasted under oxidising conditions in a multiple-hearth furnace or rotary kiln at 800—850°C for 1—2 h. Temperature control is critical because conversion of vanadium to vanadates slows markedly at ca 800°C, and the formation of Hquid phases at ca 850°C interferes with access of air to the mineral particles. During roasting, a reaction of sodium chloride with hydrous siUcates, which often are present in the ore feed, yields HCl gas. This is scmbbed from the roaster off-gas and neutralized for pollution control, or used in acid-leaching processes at the mill site. 

Perhaps the most important components of reactor solids are those that are generated during processing rather than those that are derived from inert minerals (quartz, clays) and macerals (fu-sinites, etc.) in the feed coal (74). The retention of these formed materials is more difficult to predict from the characteristics of the feed and, hence, control in liquefaction processes. 

If individual controllers are used instead of optimal computer control, several strategies are possible. In one strategy (Lynch and Fiber, 3rd IFAC Symposium on Automation of Mining, Mineral and Metal Processing, 25-32, 1980) the slurry-pump rate is controlled to maintain sump-level constant, which results in smooth cyclone operation. The water-feed rate is ratioed to the ore-feed rate, which keeps the circulating load from oscillating. The ore-feed rate is then controlled to maintain product-particle size. 

Coal beneficiation involves a series of steps to separate the mineral matter from the combustible portion of the coal. Current coal characterization for beneficiation is usually limited to measurements of the particle specific gravity distribution (washability). It is further assumed that the properties of the coal feed stream and related mineral matter remain constant during the separation or cleaning process, but the compositions of the streams do change. These changes are important in understanding the lack of expected separations. The effects of specific mineral constituents on different unit operations are described. Better measurement and analytical systems will permit improved control of the processes and better separations. 

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