188 • Marine Minerals: Exploring Our New Ocean Frontier 



terial, production capacity of the dredge, and other 

 factors. For example, sand and gravel alone may 

 constitute the valuable mineral fraction. To be sold 

 as commercial aggregate, sand and gravel are gen- 

 erally screened to remove the undesirable very fine 

 and very coarse fractions. 



One type of size separation device in common 

 use on dredges is the trommel. A trommel is sim- 

 ply a rotating cylindrical screen, large enough and 

 strong enough to withstand the shock and abrasion 

 of thousands of tons of sand and gravel sliding and 

 tumbling through it each hour. If the material is 

 mined by a bucket dredge, the material may be dis- 

 aggregated by powerful water jets while it slides 

 downward through a rotating trommel. If the ma- 

 terial is mined by a suction dredge, it may already 

 be disaggregated but may need dewatering before 

 screening. In either case gravity plays an impor- 

 tant role, since the material must first be elevated 

 in order to slide downward through the screens. 



Classifiers are used for separating particles 

 smaller than screens can handle. Classifiers sepa- 

 rate particles according to their settling rate in a 

 fluid. One type in common use is the hydrocyclone. 

 In this type of classifier, a mixture of ore and water 

 is pumped under pressure into an enclosed circu- 

 lar chamber, generating a centrifugal force. Sepa- 

 ration takes place as the heavier materials fall and 

 are discharged from the bottom while the lighter 

 particles flow out the top. Hydrocyclones are me- 

 chanically simple, require little space, and are in- 

 expensive. Most offshore tin, diamond, and gold 

 mining operations separate material by screening 

 and/or cy cloning as a first step in mineral recovery. 



Following size separation, gravity separation 

 techniques are used to concentrate most of the 

 minerals in this category. By gravity, the valuable 

 heavier minerals are separated from the lighter, less 

 valuable or worthless constituents of the ore. Proc- 

 essing by gravity concentration takes advantage of 

 the differences in density among materials. Several 

 different technologies have been developed, includ- 

 ing jigs, spirals, sluices, cones, and shaking tables. '^ 



Jigging is the action of sorting heavier particles 

 in a pulsating water column. Using either air pres- 

 sure or a piston, the pulsations are imparted to an 



'Ibid. 



introduced ore-water slurry. This action causes the 

 heavier minerals to sink to the bottom, where they 

 are drawn off. Lighter particles are entrained in 

 the cross-flow and discharged as waste. Secondary 

 or tertiary jigs may be used for further concentra- 

 tion. Several different types of jig have been de- 

 veloped, including the circular jig, which has been 

 used extensively on offshore tin dredges in South- 

 east Asia. Jigs also have been used successfully off- 

 shore to process alluvial gold and diamonds. For 

 example, they have proved effective in eliminat- 

 ing 85 to 90 percent of the waste material from tin 

 ore (cassiterite) in Indonesia and from gold ore in 

 tests near Nome, Alaska. 



Some jigs may be sensitive to the rolling and 

 pitching motion of a mining dredge at sea, depend- 

 ing in part on the severity of the motion and in part 

 on their location aboard the dredge (usually high 

 above the deck to use gravity to advantage). This 

 has not been a major problem on Indonesian off- 

 shore bucket dredges, although sea conditions there 

 are not as rough as in other parts of the world. De- 

 sign of dredges for less rolling motion and for re- 

 duced sensitivity to wind forces (e.g., by placing 

 the processing plant and machinery below the 

 waterline) would alleviate this problem. Lower pro- 

 file dredges could be designed without much diffi- 

 culty, provided economic incentives existed to do 

 so. 



A simple gravity device for concentrating some 

 placer minerals onshore is a riffle box for sluicing 

 material. Although neither well understood nor 

 very efficient, sluicing is the one of the oldest types 

 of processing technology for concentrating alluvial 

 gold or tin. In addition to their simplicity, sluices 

 are rugged, passive, and inexpensive. Although 

 sluices have not been used offshore, they might be 

 utilized to beneficiate ore of low-value heavy 

 minerals such as ilmenite or chromite. 



Many other types of gravity separation devices 

 are used onshore to separate inert heavy minerals 

 from mixtures of ore and water. The most com- 

 mon are spirals (e.g., Humphrey's spirals) and cy- 

 clones. Spirals (figure 5-15) are used extensively 

 to concentrate ilmenite, rutile, zircon, monazite, 

 chromite, and magnetite from silicate sands of 

 dunes and ancient shorelines. The effectiveness of 

 spirals mounted on platforms subject to wave mo- 

 tions is not well known, but spirals have been used 



