CHAPTER XXV 

 DREDGING FOR METALS 



DREDGES are used for mining precious metals, especially gold, 

 tin, diamonds and platinum. At first dredges were used only when 

 metals were recovered from the bottoms of rivers, but it was found 

 that by excavating a small pond or lagoon and placing a dredge in 

 it, alluvial deposits could be mined at a low cost by dredges. This 

 is now done in many parts of the world. 



Tin Dredging. For tin dredging, hydraulic dredges are used. 

 Much of this class of mining is done in Australia and New Zealand. 

 Elevator or ladder dredges can be used, and when gold and tin are 

 dredged together they are sometimes used, but with difficulty, as 

 the specific gravity of the two metals varies much. One authority 

 states that the ladder dredge for stream tin is efficient, but he has 

 little faith in it when there is much overburden. 



Platinum Dredging. Dredging for platinum is done in the 

 Urals in Russia and also in Siberia. Ladder or elevator dredges 

 have been used for some years. Those first used were machines 

 bought second hand that had been engaged in excavating the Kiel 

 Canal. These dredges made some money, especially when the price 

 of platinum was high, owing to the cheap labor that could be obtained, 

 but as they were not built for mining purposes they were poorly 

 suited to the work. Other dredges of the ladder type, designed 

 for the purpose, have been built and are operated much cheaper. 

 The dredges are used to excavate the overburden and also the gravel 

 containing the platinum. For this purpose two dredges are some- 

 times used together. The overburden being deep, two or three 

 times as much material must be handled by the dredge as when 

 working the gravel. Accordingly a large capacity dredge is used 

 for the overburden and a small one for the gravel. In this way 

 the two classes of work are done simultaneously. During the year 

 1908, 64 dredges were worked in this business. 



Diamond Dredging. Ladder dredges are also used for diamond 

 mining. The dredges save diamonds on its screens and jigs, which 



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