Ch. 5— Mining and At-Sea Processing Technologies • 189 



Figure 5-15.— Operating Principles of Three 

 Placer Mineral Separation Techniques 



a) Spiral separator 



b) Magnetic separator 



Rotating steel drum 



Fixed assembly 

 of permanent magnets 



Pulp 



c) Electro-dynamic separator 



go o 



Non-conductors 



Middlings 



Conductors 



Gravity separation using spirals may be adapted for offshore 

 use in some circumstances. Magnetic and electro-dynamic 

 separation will most likely be done on land. 



SOURCE: E.G. Kelley and D.J. Spottiswood, Introduction to Mineral Processing 

 (New York: John Wiley & Sons, 1982). 



successfully for sample concentration on board ship. 

 In operation, an ore-water slurry is introduced at 

 the top of the spiral. As the slurry spirals down- 

 ward, the lighter minerals are thrown to the out- 

 side by centrifugal force, while the heavy minerals 

 concentrate along the inner part of the spiral. The 

 heavy minerals are split from the slurry stream and 

 saved. Spirals have lower rates of throughput than 

 jigs. Moreover, more space would be required to 

 process an equal volume of minerals, and spirals 

 are unsuited for separating particles larger than 

 about one-quarter inch. 



Another form of heavy mineral processing that 

 may have applications offshore is heavy media sep- 

 aration. This gravity separation technique uses a 

 dense material in liquid suspension (the heavy 

 medium) to separate heavy minerals from lighter 

 materials. The "heavies" sink to the bottom of the 

 heavy medium, while lighter materials, such as sili- 

 cates, float away. The heavy liquid is then recir- 

 culated. This technique has been used effectively 

 offshore to recover diamonds. However, it is ex- 

 pensive and its use may contaminate seawater. 



Initial "wet" concentration at sea results in a 

 primary concentrate. Much of the technology for 

 size classification and gravity sepau-ation of minerals 

 appears to be adaptable for use at sea for making 

 primary concentrates without major technological 

 problems. For further preparation for sale, concen- 

 trates of heavy minerals are usually dried and sep- 

 arated on shore. For example, ilmenite and magne- 

 tite are considered impurities in tin ore and must 

 be eliminated. Producing heavy mineral concen- 

 trates for final sale may also involve further grav- 

 ity separation, drying in kilns, and/or elaborate 

 magnetic and electrostatic separation operations. 



Magnetic separation is possible for those minerals 

 with magnetic properties (figure 15-5). For exam- 

 ple, magnetite may be separated from other heavy 

 minerals using a low-intensity magnetic separation 

 technique. Ilmenite or other less strongly magnetic 

 minerals may be separated from nonmagnetic min- 

 erals using a high-intensity technique. Separation 

 at sea of strongly magnetic minerals is possible, but 

 separation of minerals with small differences in 

 magnetic susceptibility may have to be done on 

 land. Magnetite has the highest magnetic suscep- 

 tibility. In decreasing order of susceptibility are il- 

 menite and chromite; epidote and xenotime; apa- 

 tite, monazite, and hematite; and staurolite. 



