Ch. 5— Mining and At-Sea Processing Technologies * 177 



Figure 5-8.— Airlift Suction Dredge Configuration 



Compressor 



Water line 



/ A Water 



f ^^ pump 



Airlift dredges may be applicable for some seabed deposits 

 300 feet or more below the ocean surface. Airlift dredging 

 has been used on a pilot scale to lift manganese nodules from 

 about 15,000 feet. 



SOURCE: Office of Technology Assessment, 1987. 



tions of airlifts for mining offshore minerals may 

 be considered for depths between 300 and 16,000 

 feet. Suction and air delivery lines can be handled 

 with techniques readily adapted from the petroleum 

 industry; the problem of platform motion in re- 

 sponse to long period waves can be overcome by 

 adapting motion compensation systems used in the 

 petroleum industry; and seabed material can be dis- 

 aggregated at the suction intake by high-pressure 

 water jets or by hydraulically driven mechanical 

 cutters. 



Grab Dredges 



Grab dredging is the mechanical action of cut- 

 ting or scooping material from the seabed in finite 

 quantities and lifting the filled "grab" container 

 to the ocean surface. Grab dredging takes place in 

 a cycle: lower, fill, lift, discharge, and again lower 



the grab bucket. Clamshell, dragline, dipper, and 

 backhoe dredges are examples of this technology 

 (figure 5-9). Clamshells and draglines are widely 

 used for dredging boulders or massive rock frag- 

 ments broken by explosives and for removing over- 

 burden from coal and other stratified mineral de- 

 posits. The clamshell and dragline buckets are 

 lowered and lifted with flexible steel cables. Vari- 

 ants of clamshell dredging have been used in Thai- 

 land to mine tin in Phuket Harbor and in Japan 

 to mine iron sands in Ariake Bay. In the late 1960s, 

 Global Marine, Inc., used a clamshell dredge for 

 pilot mining of gold-bearing material from depths 

 of 1,000 feet near Juneau, Alaska. Variants of 

 dragline dredges have been used since the late 19th 

 century to recover material from the deep seafloor. 



With appropriate winch configurations for han- 

 dling large amounts of cable and large buckets, grab 

 dredging is similar to the traditional technologies 

 used to hoist material from deep underground 

 mines (e.g., in South Africa, where it is economi- 

 cally feasible to hoist gold ores from 12,000 feet be- 

 low the ground surface). Most aspects of clamshell 

 dredging technology, including motion compensa- 

 tion for working on a moving platform at sea, have 

 been developed and proven by either the mining 

 or petroleum industry and are readily available for 

 adaptation to offshore mining. 



Dipper and backhoe dredges are designed for use 

 on land (figure 5-9). They may be placed on float- 

 ing pontoons for offshore dredging but are limited 

 to shallow-water applications. Backhoes especially 

 can be easily adapted to mining in protected shal- 

 low water. Commercial off-the-shelf backhoes with 

 a maximum reach of about 30 feet and buckets with 

 capacities of up to 3 cubic yards are readily avail- 

 able for gold or tin placer mining in protected envi- 

 ronments. Backhoes mounted on walking platforms 

 are conceivable for excavation in shallow surf zones. 

 Backhoe mining is limited by depth of reach, small 

 capacity, and the inability of the operator to see 

 the cutting action of the bucket below water. Dip- 

 per dredges are widely used to mine stratified 

 mineral deposits (e.g., coal and bauxite) on land, 

 but their unique action (figure 5-9) restricts offshore 

 applications to shallow water. As dredged material 

 using grab, dipper, and backhoe dredges is raised 

 through the water column, the material is washed, 

 which may not be desirable in mining. 



