| Abstract |
With a greater awareness of environmental issues, there has been a move. towards more efficient use of the world's natural resources. Until recently, little use has been made of the heat contained in the geothermal waterfollowing steam separation although the heat energy contained in the separated water can be greater than that contained in the steam. The cuwent trend of installation of binay plants to produce electricity is the first major utilisation of this heat. Downstream of binay plants, utilisation of the available heat energy is best accomplished by directly using the heat content of the water. One of the main reasons why this has not been practised in the past is the problem of silica deposition. Research has shown that this problem can possibly be alleviated by either treatment of the water to stabilise the silica, or total removal of the silica. One possibility could be to temporarily stabilise the silica and extract the heat through a heat exchanger thus transferring the heat energy to ajluid that will not cause scaling problems. As well as the heat energy, the separated water also contains a large number of dissolved minerals. Research is actively been pursued to produce a commercial product from the silica, and once this has been removed, the possibility exists to extract other commercially useful minerals. Possibly, the most valuable constituent is lithium. Technology does not cuwently exist at a commercial stage to accomplish lithium recovey, but laboratoy work has shown that it is at least feasible. Similarly, the water contains quite large quantities of boron and laboratoy tests have shown that this can also be extracted. A nother possibility is the extraction of arsenic. A lthough of negligible value, it has the advantage of reducing the environmental risk. The major component ofgeothermal water is common salt, and it could also be economically extracted., Other minerals for possible extraction are gold, silver,andpotassium chloride, and once all of these minerals have been extracted, it may be possible to use the water itseyfor low grade uses. As well as useful minerals, the geothermal water also contains environmentally sensitive elements and compounds. The two principal options that exist for the disposal of the water are reinjection and surj5ace disposal. A lthough reinjection was seen initially as usejicl for maintaining reservoir pressure, experience at a number of geothermal fields around the world has suggested that the beneficial effects of reinjection are outweighed by the detrimental effects due to cooling of the reservoir. Currently, reinjection is practised mainly to avoid the environmental issues connected with surface disposal. Treatment of the geothermal water to extract the environmentally sensitive components becomes much more feasible iy silica has been previously removed. Therefore, iymineral extraction is envisaged, continuing disposal of the treated waste water to the surj5ace could be possible. Since many of the mineral extraction processes, heat uses and environmental controls are inter-related, future planning should em brace the concept of a total, integrated development of the resource. |