Mining the hydrosphere

Abstract Rapid technological progress over the past two decades has significantly lowered the cost of water desalination and has spurred an impressive growth of this industry. About half of the desalination capacity uses seawater, the other half uses continental brackish water. Most of the desalted water is consumed for domestic and municipal purposes. However, some of it, especially that derived from brackish water, is also competitive for irrigation of high-value crops, and for some industrial purposes, particularly in water-deficient regions. In addition to fresh water, at present only halite, magnesium, and bromine are commercially obtained from seawater. These commodities plus sodium carbonate (trona), sodium sulfate, I, Li, B, and potash are also produced from natural brines. Prior attempts to obtain potash, U, Au, and other mineral commodities from seawater failed because the market value of the recovered products was too low to cover the capital and operating costs of processing plants exclusively dedicated to recover them separately. The economics are more favorable if these and/or other elements or compounds are obtained as byproducts of seawater desalination, especially when combined with cogeneration of electricity. Under these circumstances the major capital and operating costs for pumping seawater and for disposing of the reject brine are absorbed mostly by the proceeds from freshwater production. The byproducts need only to pay for the additional recovery processes. One advantage of this strategy is to reduce the environmental impact of reject brine disposal. Another is to reduce the environmental, safety, and health impacts of land-based mining. Furthermore, obtaining nonmetallic mineral commodities from seawater at a number of localities scattered over the Earth can significantly reduce their transportation costs, which is a major proportion of their cost to nations lacking these resources. This is particularly pertinent for common salt (halite), potash, sodium carbonate (trona), sodium sulfate, S, and gypsum. These compounds, plus B, Cl, Calcium chloride, Li, and Sr (perhaps also F and U) are the best candidates for recovery from seawater because their value per ton of seawater is greater than that of other products. Further research aimed at recovering the aforementioned elements and compounds from seawater is justified and recommended. Given the many uncertainties involved, it is beyond the scope of this paper to present specific flow sheets and estimates of capital and operating costs for byproduct recovery. Rather, the purpose of this contribution is to provide a general overview of the potential benefits and problems, so that future research can be directed more fruitfully to the recovery of certain sets of elements or compounds under specific circumstances. Once a mineral commodity can be economically obtained from seawater, there is no further need to mine it on land from lower grade, deeper or more distant ore deposits (or to mine it in ecologically sensitive areas). Current producers need not excessively fear the proposed new supplies because in the past high transportation costs often prevented their nonmetallic commodities from reaching the distant potential markets that would be served by many byproduct-producing seawater processing plants. In addition, population growth and rising standards of living may well absorb much of the feared overcapacity in their spheres of influence. For traditional metals, such as Fe, Al, Cu, Pb, Zn, Au, and Ag, byproduct recovery from seawater desalination appears to be out of reach for a long time.