the formula (Zn, Fe)S and thus always contains iron. Commonly both are true: for example, copper is generally smelted from a mixture of sulfides physically concentrated by flotation. These include copper-only sulfides, copper-iron sulfides, iron-only sulfides, copper-iron arseno-sulfides, and selenides. Again, therefore, iron is always present and sometimes cobalt and nickel must also be dealt with. Side reactions are nearly always undesirable. They cause reagent loss and/or the creation of undesirable by-products and contaminants. Such by-products can have deleterious effects on the extraction procedure, or even on the very processing equipment itself, as when corrosive compounds are made. Such chemical tangles are why it is so cost-effective to let nature do as much of the separation as possible first; i.e., why "ores," anomalous concentrations of desired elements in easily separable forms, are worth seeking out in the first place. Examples of side reactions include cyanide loss in heap leaching of low-grade gold ore. To extract the extremely fine grains of native gold, which can amount to as little as about 0.6 ppm, broken rock is stacked into flatheaps about 10m high and a cyanide solution sprinkled over them. Gold and silver readily form stable cyanide complexes, which can then be extracted from the solution exiting below. However, cyanide losses, through adsorption by organic matter or reactions with other metals, such as copper, can make such leaching uneconomic, so that the gold content alone may not determine whether a given deposit is economic (cf. Bray, 1941). On the Moon, an important side reaction that has been little noticed is the formation of sulfuric acid if hydrogen is extracted or used as a reactant. Regolith contains a little troilite (FeS), which at moderate temperature will be oxidized by water vapor to yield H2SO4. A casual perusal of any extractive metallurgy text (e.g., Roscnqvist, 1983) will also demonstrate the degree to which side reactions greatly complicate the extraction of metals. Multiple steps of extraction and refining are nearly always necessary to purify a metal before it is usable. Another complication of separation processes, especially chemical processes, is their tendency to oscillate uncontrollably, as is typical of an unstable system. Processing control is made much easier by using a series of simple separations, rather than a single or a few complicated processes. Each such simple step is much less likely to oscillate, because it has a longer time constant and its state can be sensed adequately in real time. Small process steps are easier to sense and control, and any oscillations can be confined to single steps rather than the whole process. As will be discussed below, the heterogeneity of natural materials guarantees that the feedstock to any process will vary, and such variation must be dealt with in real time. This phenomenon is another motivation for using multiple steps in an extraction procedure. Heterogeneity Natural materials are also heterogeneous, both in composition and in physical
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