The plant also must be as simple as possible, and must focus on specific elements to the exclusion of others. Finally, a totally automated facility seems unattainable at this point, at least without major breakthroughs in sensors and artificial intelligence. The cases described below result from unforeseen consequences of underappreciated complexities in the material on which processing was attempted. They have salutary lessons for the extension of resource extraction to other worlds. Concentrating the Ore Mineral Even the most valuable mineral deposit is rarely sufficiency high grade to be processed directly without first forming a high-grade mineral concentrate from it by mineral dressing. Terrestrial mineral dressing ordinarily concentrates a single mineral or element, with the bulk of the rock mined (the "gangue") discarded. The gangue may even contain grains of the desired ore mineral that were too small or too dispersed to be economic. Only the relatively pure ore mineral is included in the mineral concentrate. Ore mineral recovery can be based on contrast in physical and/or chemical properties between the desired mineral or element and the host rock, but physical processes are generally cheaper. Since several mineral separation processes are usually used together or in sequence, they have a profound economic effect on the cost of the mineral or element. Physical properties of interest are (e.g., Richards - Locke, 1940; Taggart, 1954): Hardness or softness Tenacity, brittleness or friability. Structure and fracture. Friction. Aggregation. Color and Luster. Density. Electroconductivity. Magnetic susceptibility. Decrepitation by heat Surface properties: Greasiness. Adhesion. Wettability. Contact angle. Polarity. Surface tension. Chemical processes that affect physical properties are (e.g., Richards - Locke, 1940):
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