Gold from sea water (new technique)
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Sea water contains about 0.1-2 mg/tonne of gold dissolved in water (average 1 mg/tonne). But considering the amount of seawater available, it is a really huge goldmine! Theoretically fine, but problems were practical (which prevented profitable extraction till now). This can however become possible with the old electrolysis technique, with the only difference that the voltage difference between the electrodes must be maintained slightly less than the minimum potential difference required for electrolysis of water (yes, there is a minimum pot. difference, say 1.48 volts, below which water won't be hydrolysed. But since gold lies below hydrogen in electrochemical series, it will get deposited on the cathode!). Since it is impractical to pump millions of gallons of water, it is more practical to move the electrodes over vast regions of oceans.
With a slight modification, the propellors of ships can be designed to form the electrodes! Each of the 3 blades will be a stack of 3 blades (like a sandwich) with the sandwiched blade maintained +ve and the other two forming cathode (of course they won't be touching each other. There will be a gap of a few cm between each blade, supported by rubber/cork). The tilt of each blades will be much less than conventional propellor, so that it makes much more revolution per advancement, and hence scan the volume of water more effectively. It is practical to make each blade 1.7 metre in length, so that cross sectional area of circle formed on revolution of blades will be 10 metre square. This will scan 10 tonne of water per 1 metre moved by the ship. Considering that efficiency of extraction is only 0.1 mg/tonne, it comes to 1 mg/metre of distance covered (or 1 gram per k.m. or 1 k.g gold per 1000 k.m.) So, this may not be profitable if ship is designed only for gold hunt. But it can be a real bonus for commercial ships which has to cover thousands of k.m. anyway.